diff --git a/crates/clawhdf5-format/src/addr.rs b/crates/clawhdf5-format/src/addr.rs
index ec25e3d..09c2cef 100644
--- a/crates/clawhdf5-format/src/addr.rs
+++ b/crates/clawhdf5-format/src/addr.rs
@@ -23,6 +23,19 @@ pub fn to_usize(value: u64) -> Result<usize, FormatError> {
     usize::try_from(value).map_err(|_| too_large(value))
 }
 
+/// A count or offset into an in-memory buffer (a codec's progress counter,
+/// a size the writer computed from data it holds) as a `usize`, saturating
+/// at `usize::MAX` instead of truncating.
+///
+/// For values that are bounded by the length of something in memory, so
+/// always fit; if one ever did not, a saturated index fails its bounds check
+/// or allocation instead of silently addressing the wrong bytes. A value
+/// read from the file uses [`to_usize`].
+#[inline]
+pub fn saturating_usize(value: u64) -> usize {
+    usize::try_from(value).unwrap_or(usize::MAX)
+}
+
 #[cold]
 #[inline(never)]
 fn too_large(value: u64) -> FormatError {
@@ -42,6 +55,15 @@ mod tests {
         assert_eq!(to_usize(usize::MAX as u64), Ok(usize::MAX));
     }
 
+    #[test]
+    fn saturating_conversion_never_wraps() {
+        assert_eq!(saturating_usize(0), 0);
+        assert_eq!(saturating_usize(0x1234), 0x1234);
+        assert_eq!(saturating_usize(usize::MAX as u64), usize::MAX);
+        // Past usize::MAX (32-bit targets) or at u64::MAX: saturates.
+        assert_eq!(saturating_usize(u64::MAX), usize::MAX);
+    }
+
     #[test]
     fn values_past_usize_max_are_an_error_not_truncated() {
         // Only reachable where usize is narrower than u64; on a 64-bit host
diff --git a/crates/clawhdf5-format/src/btree_v1.rs b/crates/clawhdf5-format/src/btree_v1.rs
index b652dcc..0e9b7fe 100644
--- a/crates/clawhdf5-format/src/btree_v1.rs
+++ b/crates/clawhdf5-format/src/btree_v1.rs
@@ -3,6 +3,7 @@
 #[cfg(not(feature = "std"))]
 use alloc::vec::Vec;
 
+use crate::addr::to_usize;
 use crate::error::FormatError;
 
 /// A parsed B-tree v1 node.
@@ -164,7 +165,12 @@ fn collect_symbol_table_nodes_inner(
         return Err(FormatError::NestingDepthExceeded);
     }
 
-    let node = BTreeV1Node::parse(file_data, btree_address as usize, offset_size, length_size)?;
+    let node = BTreeV1Node::parse(
+        file_data,
+        to_usize(btree_address)?,
+        offset_size,
+        length_size,
+    )?;
 
     if node.node_type != 0 {
         return Err(FormatError::InvalidBTreeNodeType(node.node_type));
diff --git a/crates/clawhdf5-format/src/btree_v2_write.rs b/crates/clawhdf5-format/src/btree_v2_write.rs
index 795fb80..38fa7c1 100644
--- a/crates/clawhdf5-format/src/btree_v2_write.rs
+++ b/crates/clawhdf5-format/src/btree_v2_write.rs
@@ -6,6 +6,7 @@
 //! libhdf5 uses (`H5B2__hdr_init`) and the reader decodes pointers with, so
 //! the pointer widths the writer encodes are the ones every reader expects.
 
+use crate::addr::saturating_usize;
 #[cfg(not(feature = "std"))]
 use alloc::{format, vec, vec::Vec};
 
@@ -105,7 +106,9 @@ pub(crate) fn build_btree_v2(
         first_node: addr + hdr_len as u64,
         nodes: Vec::new(),
     };
-    let root = (n > 0).then(|| w.node(depth, 0, n as usize)).transpose()?;
+    let root = (n > 0)
+        .then(|| w.node(depth, 0, saturating_usize(n)))
+        .transpose()?;
 
     let mut out = Vec::with_capacity(hdr_len + w.nodes.len() * p.node_size as usize);
     out.extend_from_slice(b"BTHD");
@@ -201,7 +204,7 @@ impl TreeWriter<'_> {
                 "cannot spread {n} B-tree v2 records over {k} children at depth {depth}"
             )));
         }
-        let k = k as usize;
+        let k = saturating_usize(k);
         let in_children = n - (k - 1);
         let (base, extra) = (in_children / k, in_children % k);
 
diff --git a/crates/clawhdf5-format/src/chunk_index.rs b/crates/clawhdf5-format/src/chunk_index.rs
index c307572..706bad0 100644
--- a/crates/clawhdf5-format/src/chunk_index.rs
+++ b/crates/clawhdf5-format/src/chunk_index.rs
@@ -18,6 +18,7 @@ use alloc::collections::BTreeMap;
 #[cfg(feature = "std")]
 use std::collections::HashMap;
 
+use crate::addr::to_usize;
 use crate::chunk_cache::ChunkCoord;
 use crate::chunked_read::ChunkInfo;
 
@@ -167,7 +168,15 @@ impl ChunkLayout {
 
         for (_coord, ci) in index.iter() {
             let coord: ChunkCoord = ci.offsets.iter().take(rank).copied().collect();
-            let chunk_offsets: Vec<usize> = coord.iter().map(|&o| o as usize).collect();
+            // `ds_dims` are `usize`: a chunk at an offset past `usize::MAX`
+            // (only on a 32-bit target) lies outside the dataset.
+            let Ok(chunk_offsets) = coord
+                .iter()
+                .map(|&o| to_usize(o))
+                .collect::<Result<Vec<usize>, _>>()
+            else {
+                continue;
+            };
 
             let copies = if rank == 0 {
                 // Scalar dataset — single copy
diff --git a/crates/clawhdf5-format/src/chunked_read.rs b/crates/clawhdf5-format/src/chunked_read.rs
index 6dda3cb..e249168 100644
--- a/crates/clawhdf5-format/src/chunked_read.rs
+++ b/crates/clawhdf5-format/src/chunked_read.rs
@@ -6,6 +6,7 @@ extern crate alloc;
 #[cfg(not(feature = "std"))]
 use alloc::{format, vec, vec::Vec};
 
+use crate::addr::to_usize;
 #[cfg(feature = "std")]
 use crate::chunk_cache::{CacheAlignedBuffer, ChunkCache};
 use crate::data_layout::DataLayout;
@@ -265,7 +266,7 @@ fn fill_from_chunks(
             )));
         }
         let offsets = &c.offsets[..rank];
-        let c_addr = c.address as usize;
+        let c_addr = to_usize(c.address)?;
         let size = c.chunk_size as usize;
         ensure_len(file_data, c_addr, size)?;
         let raw = &file_data[c_addr..c_addr + size];
@@ -825,7 +826,7 @@ fn parse_chunk_node(
         return Err(FormatError::NestingDepthExceeded);
     }
 
-    let offset = btree_address as usize;
+    let offset = to_usize(btree_address)?;
     let os = offset_size as usize;
 
     // Parse B-tree v1 header
@@ -945,7 +946,8 @@ pub fn generate_implicit_chunks(
     }
     let total_chunks: u64 = num_chunks_per_dim.iter().product();
 
-    let mut chunks = Vec::with_capacity(total_chunks as usize);
+    // A capacity hint only (a count past `usize::MAX` could not be pushed).
+    let mut chunks = Vec::with_capacity(usize::try_from(total_chunks).unwrap_or(0));
     for linear_idx in 0..total_chunks {
         let mut offsets = vec![0u64; rank];
         let mut remaining = linear_idx;
@@ -993,7 +995,7 @@ fn read_btree_v2_chunks(
     use crate::btree_v2::{BTreeV2Header, collect_btree_v2_records};
 
     let bad = |what: &str| FormatError::ChunkedReadError(format!("B-tree v2 chunk index: {what}"));
-    let header = BTreeV2Header::parse(file_data, addr as usize, offset_size, length_size)?;
+    let header = BTreeV2Header::parse(file_data, to_usize(addr)?, offset_size, length_size)?;
     let rank = chunk_dims.len();
     let os = offset_size as usize;
     let record_size = header.record_size as usize;
@@ -1122,7 +1124,11 @@ pub fn list_chunks(
 
     // Both v3 and v4 include element size as last dim (rank+1)
     let (rank, chunk_dims) = chunk_geometry(chunk_dimensions, version, dataspace, elem_size)?;
-    let ds_dims: Vec<usize> = dataspace.dimensions.iter().map(|&d| d as usize).collect();
+    let ds_dims: Vec<usize> = dataspace
+        .dimensions
+        .iter()
+        .map(|&d| to_usize(d))
+        .collect::<Result<_, _>>()?;
 
     // Collect chunks based on version and index type
     let mut chunks = match (version, chunk_index_type) {
@@ -1158,7 +1164,7 @@ pub fn list_chunks(
             // Fixed Array — use spatial chunk dims only
             let spatial_chunk_dims: &[u32] = &chunk_dimensions[..rank];
             let header =
-                FixedArrayHeader::parse(file_data, addr as usize, offset_size, length_size)?;
+                FixedArrayHeader::parse(file_data, to_usize(addr)?, offset_size, length_size)?;
             read_fixed_array_chunks(
                 file_data,
                 &header,
@@ -1174,7 +1180,7 @@ pub fn list_chunks(
             // Extensible Array — use spatial chunk dims only
             let spatial_chunk_dims: &[u32] = &chunk_dimensions[..rank];
             let header =
-                ExtensibleArrayHeader::parse(file_data, addr as usize, offset_size, length_size)?;
+                ExtensibleArrayHeader::parse(file_data, to_usize(addr)?, offset_size, length_size)?;
             read_extensible_array_chunks(
                 file_data,
                 &header,
@@ -1349,7 +1355,11 @@ pub(crate) fn read_chunked_full<O>(
         // dimension the total is 0 even if other dimensions are huge.
         return Ok(output);
     }
-    let ds_dims: Vec<usize> = dataspace.dimensions.iter().map(|&d| d as usize).collect();
+    let ds_dims: Vec<usize> = dataspace
+        .dimensions
+        .iter()
+        .map(|&d| to_usize(d))
+        .collect::<Result<_, _>>()?;
     let placer = ChunkPlacer::new(&chunk_dims, &ds_dims, elem_size);
     let chunk_total_bytes = checked_chunk_byte_len(&chunk_dims, elem_size)?;
     // Chunks are cached only when the whole dataset fits: pushing a larger
@@ -1603,7 +1613,11 @@ pub fn read_chunked_data_sweep(
     check_chunk_element_size(layout, datatype, offset_size)?;
     let elem_size = datatype.type_size() as usize;
     let (rank, chunk_dims) = chunk_geometry(chunk_dimensions, version, dataspace, elem_size)?;
-    let ds_dims: Vec<usize> = dataspace.dimensions.iter().map(|&d| d as usize).collect();
+    let ds_dims: Vec<usize> = dataspace
+        .dimensions
+        .iter()
+        .map(|&d| to_usize(d))
+        .collect::<Result<_, _>>()?;
 
     // The per-file cache is shared across datasets (and threads); every
     // lookup is keyed by this dataset's chunk-index address, so another
@@ -1659,7 +1673,7 @@ pub fn read_chunked_data_sweep(
             cached
         } else {
             // Decompress from file
-            let c_addr = chunk_info.address as usize;
+            let c_addr = to_usize(chunk_info.address)?;
             let size = chunk_info.chunk_size as usize;
             ensure_len(file_data, c_addr, size)?;
             let raw_chunk = &file_data[c_addr..c_addr + size];
@@ -1682,8 +1696,8 @@ pub fn read_chunked_data_sweep(
             .offsets
             .iter()
             .take(rank)
-            .map(|&o| o as usize)
-            .collect();
+            .map(|&o| to_usize(o))
+            .collect::<Result<_, _>>()?;
 
         if rank == 0 {
             let copy_len = decompressed.len().min(output.len());
@@ -1743,7 +1757,11 @@ pub fn read_chunked_data_indexed(
     check_chunk_element_size(layout, datatype, offset_size)?;
     let elem_size = datatype.type_size() as usize;
     let (rank, chunk_dims) = chunk_geometry(chunk_dimensions, version, dataspace, elem_size)?;
-    let ds_dims: Vec<usize> = dataspace.dimensions.iter().map(|&d| d as usize).collect();
+    let ds_dims: Vec<usize> = dataspace
+        .dimensions
+        .iter()
+        .map(|&d| to_usize(d))
+        .collect::<Result<_, _>>()?;
 
     // Chunk index and assembly plan for this dataset, built on first access
     // and kept per dataset (keyed by chunk-index address) in the shared cache.
@@ -1776,7 +1794,7 @@ pub fn read_chunked_data_indexed(
         if let Some(cached) = cache.get_decompressed_in(addr, coord) {
             chunk_buffers.push(cached);
         } else {
-            let c_addr = *file_offset as usize;
+            let c_addr = to_usize(*file_offset)?;
             let size = *file_size as usize;
             ensure_len(file_data, c_addr, size)?;
             let raw_chunk = &file_data[c_addr..c_addr + size];
diff --git a/crates/clawhdf5-format/src/chunked_write.rs b/crates/clawhdf5-format/src/chunked_write.rs
index 2a8bd09..cf5c958 100644
--- a/crates/clawhdf5-format/src/chunked_write.rs
+++ b/crates/clawhdf5-format/src/chunked_write.rs
@@ -3,6 +3,7 @@
 #[cfg(not(feature = "std"))]
 extern crate alloc;
 
+use crate::addr::saturating_usize;
 #[cfg(not(feature = "std"))]
 use alloc::{format, vec, vec::Vec};
 
@@ -414,18 +415,18 @@ pub fn split_into_chunks(
     // Dataset strides (row-major)
     let mut ds_strides = vec![1usize; rank];
     for i in (0..rank.saturating_sub(1)).rev() {
-        ds_strides[i] = ds_strides[i + 1] * shape[i + 1] as usize;
+        ds_strides[i] = ds_strides[i + 1] * saturating_usize(shape[i + 1]);
     }
 
     // Chunk strides
     let mut chunk_strides = vec![1usize; rank];
     for i in (0..rank.saturating_sub(1)).rev() {
-        chunk_strides[i] = chunk_strides[i + 1] * chunk_dims[i + 1] as usize;
+        chunk_strides[i] = chunk_strides[i + 1] * saturating_usize(chunk_dims[i + 1]);
     }
 
-    let chunk_total_elements: usize = chunk_dims.iter().map(|&d| d as usize).product();
+    let chunk_total_elements: usize = chunk_dims.iter().map(|&d| saturating_usize(d)).product();
 
-    let mut result = Vec::with_capacity(total_chunks as usize);
+    let mut result = Vec::with_capacity(saturating_usize(total_chunks));
 
     for linear_idx in 0..total_chunks {
         // Convert linear index to chunk grid coordinates
@@ -453,8 +454,8 @@ pub fn split_into_chunks(
                 let coord_in_chunk = remaining_idx / chunk_strides[d];
                 remaining_idx %= chunk_strides[d];
 
-                let global_coord = offsets[d] as usize + coord_in_chunk;
-                if global_coord >= shape[d] as usize {
+                let global_coord = saturating_usize(offsets[d]) + coord_in_chunk;
+                if global_coord >= saturating_usize(shape[d]) {
                     out_of_bounds = true;
                     break;
                 }
@@ -1036,7 +1037,7 @@ impl ChunkIndexPlan {
                     Ok(Self::SingleChunk)
                 } else {
                     let grid = ChunkGrid::fixed_array(shape, Some(max), chunk_dims)?;
-                    Ok(Self::FixedArray(grid, nslots as usize))
+                    Ok(Self::FixedArray(grid, saturating_usize(nslots)))
                 }
             }
             1 => Ok(Self::ExtensibleArray(ChunkGrid::extensible_array(
@@ -1251,7 +1252,7 @@ pub fn write_selection_to_buffer(
             let rank = dims.len();
             let mut ds_strides = vec![1usize; rank];
             for i in (0..rank.saturating_sub(1)).rev() {
-                ds_strides[i] = ds_strides[i + 1] * dims[i + 1] as usize;
+                ds_strides[i] = ds_strides[i + 1] * saturating_usize(dims[i + 1]);
             }
 
             let mut src_offset = 0usize;
@@ -1301,7 +1302,7 @@ pub fn write_selection_to_buffer(
                                 buffer,
                                 new_data,
                                 src_offset,
-                                current_ds_offset + coord as usize * ds_strides[d],
+                                current_ds_offset + saturating_usize(coord) * ds_strides[d],
                             );
                         }
                     }
@@ -1328,14 +1329,14 @@ pub fn write_selection_to_buffer(
             let rank = dims.len();
             let mut ds_strides = vec![1usize; rank];
             for i in (0..rank.saturating_sub(1)).rev() {
-                ds_strides[i] = ds_strides[i + 1] * dims[i + 1] as usize;
+                ds_strides[i] = ds_strides[i + 1] * saturating_usize(dims[i + 1]);
             }
 
             for (pi, pt) in pts.iter().enumerate() {
                 let flat: usize = pt
                     .iter()
                     .zip(ds_strides.iter())
-                    .map(|(&p, &s)| p as usize * s)
+                    .map(|(&p, &s)| saturating_usize(p) * s)
                     .sum();
                 let dst = flat * elem_size;
                 let src = pi * elem_size;
diff --git a/crates/clawhdf5-format/src/data_layout.rs b/crates/clawhdf5-format/src/data_layout.rs
index 59a9065..c9b93c7 100644
--- a/crates/clawhdf5-format/src/data_layout.rs
+++ b/crates/clawhdf5-format/src/data_layout.rs
@@ -6,6 +6,7 @@ use alloc::{format, string::String, vec::Vec};
 #[cfg(feature = "std")]
 use std::string::String;
 
+use crate::addr::to_usize;
 use crate::error::FormatError;
 
 /// A single VDS (Virtual Dataset) source mapping.
@@ -207,7 +208,7 @@ pub fn parse_vds_mappings(
                     "VDS mapping shares a name with a later entry".into(),
                 ));
             }
-            Ok(idx as usize)
+            to_usize(idx)
         };
 
         let source_file = if flags & VDS_SOURCE_SAME_FILE != 0 {
@@ -320,7 +321,7 @@ impl DataLayout {
         {
             let coll = crate::global_heap::GlobalHeapCollection::parse(
                 file_data,
-                addr as usize,
+                to_usize(addr)?,
                 length_size,
             )?;
             let obj = coll.get_object(*global_heap_index as u16).ok_or(
diff --git a/crates/clawhdf5-format/src/data_read.rs b/crates/clawhdf5-format/src/data_read.rs
index ef73a04..bcf1a18 100644
--- a/crates/clawhdf5-format/src/data_read.rs
+++ b/crates/clawhdf5-format/src/data_read.rs
@@ -6,6 +6,7 @@ use alloc::{collections::BTreeMap, format, string::String, vec, vec::Vec};
 #[cfg(feature = "std")]
 use std::collections::BTreeMap;
 
+use crate::addr::to_usize;
 #[cfg(feature = "std")]
 use crate::chunk_cache::ChunkCache;
 use crate::chunked_read::read_chunked_data;
@@ -117,7 +118,7 @@ pub fn read_raw_data_zerocopy<'a>(
     dataspace: &Dataspace,
     datatype: &Datatype,
 ) -> Result<Option<&'a [u8]>, FormatError> {
-    let num_elements = dataspace.num_elements() as usize;
+    let num_elements = to_usize(dataspace.num_elements())?;
     let elem_size = datatype.type_size() as usize;
     let expected_size = num_elements.checked_mul(elem_size).ok_or_else(|| {
         FormatError::Overflow(format!(
@@ -128,7 +129,7 @@ pub fn read_raw_data_zerocopy<'a>(
     match layout {
         DataLayout::Contiguous { address, size } => {
             let addr = address.ok_or(FormatError::NoDataAllocated)?;
-            let addr = addr as usize;
+            let addr = to_usize(addr)?;
             let sz = contiguous_read_len(*size, expected_size)?;
             ensure_len(file_data, addr, sz)?;
             Ok(Some(&file_data[addr..addr + sz]))
@@ -219,7 +220,7 @@ fn read_raw_data_full_impl(
     length_size: u8,
     resolver: Option<&VdsSourceResolver>,
 ) -> Result<Vec<u8>, FormatError> {
-    let num_elements = dataspace.num_elements() as usize;
+    let num_elements = to_usize(dataspace.num_elements())?;
     let elem_size = datatype.type_size() as usize;
     let expected_size = num_elements.checked_mul(elem_size).ok_or_else(|| {
         FormatError::Overflow(format!(
@@ -239,7 +240,7 @@ fn read_raw_data_full_impl(
         }
         DataLayout::Contiguous { address, size } => {
             let addr = address.ok_or(FormatError::NoDataAllocated)?;
-            let addr = addr as usize;
+            let addr = to_usize(addr)?;
             let sz = contiguous_read_len(*size, expected_size)?;
             ensure_len(file_data, addr, sz)?;
             let mut out = crate::bulk_alloc::vec_for_bulk(sz);
@@ -582,7 +583,7 @@ pub fn extract_selection_from_buffer(
             let rank = dims.len();
             let mut ds_strides = vec![1usize; rank];
             for i in (0..rank.saturating_sub(1)).rev() {
-                ds_strides[i] = ds_strides[i + 1] * dims[i + 1] as usize;
+                ds_strides[i] = ds_strides[i + 1] * to_usize(dims[i + 1])?;
             }
 
             let mut output = Vec::with_capacity(pts.len() * elem_size);
@@ -590,8 +591,8 @@ pub fn extract_selection_from_buffer(
                 let flat: usize = pt
                     .iter()
                     .zip(ds_strides.iter())
-                    .map(|(&p, &s)| p as usize * s)
-                    .sum();
+                    .map(|(&p, &s)| Ok(to_usize(p)? * s))
+                    .sum::<Result<usize, FormatError>>()?;
                 let src = flat * elem_size;
                 if src + elem_size <= full_data.len() {
                     output.extend_from_slice(&full_data[src..src + elem_size]);
@@ -1341,7 +1342,7 @@ pub fn read_compound_fields(
             let mut fields = Vec::with_capacity(members.len());
             for m in members {
                 let field_size = m.datatype.type_size() as usize;
-                let offset = m.byte_offset as usize;
+                let offset = to_usize(m.byte_offset)?;
                 if offset
                     .checked_add(field_size)
                     .is_none_or(|end| end > elem_size)
diff --git a/crates/clawhdf5-format/src/ea_writer.rs b/crates/clawhdf5-format/src/ea_writer.rs
index f669534..de7859c 100644
--- a/crates/clawhdf5-format/src/ea_writer.rs
+++ b/crates/clawhdf5-format/src/ea_writer.rs
@@ -3,6 +3,7 @@
 #[cfg(not(feature = "std"))]
 extern crate alloc;
 
+use crate::addr::saturating_usize;
 #[cfg(not(feature = "std"))]
 use alloc::{vec, vec::Vec};
 
@@ -247,7 +248,7 @@ pub fn build_extensible_array_at(
 
     // Header (EAHD). The six statistics are, in order: super blocks, their
     // bytes, data blocks, their bytes, max index set, elements realised.
-    let mut out = Vec::with_capacity((cursor - ea_base_address) as usize);
+    let mut out = Vec::with_capacity(saturating_usize(cursor - ea_base_address));
     out.extend_from_slice(b"EAHD");
     out.push(0); // version
     out.push(client_id);
diff --git a/crates/clawhdf5-format/src/extensible_array.rs b/crates/clawhdf5-format/src/extensible_array.rs
index ba1c822..9256326 100644
--- a/crates/clawhdf5-format/src/extensible_array.rs
+++ b/crates/clawhdf5-format/src/extensible_array.rs
@@ -9,6 +9,7 @@ extern crate alloc;
 #[cfg(not(feature = "std"))]
 use alloc::{format, vec, vec::Vec};
 
+use crate::addr::to_usize;
 use crate::chunk_grid::ChunkGrid;
 use crate::chunked_read::ChunkInfo;
 use crate::error::FormatError;
@@ -451,7 +452,7 @@ pub fn read_extensible_array_chunks(
     // Parse index block (EAIB): signature(4) + version(1) + client_id(1)
     // + header address(offset_size), then the inline elements, then the
     // direct data block addresses, then the super block addresses.
-    let ib_offset = header.index_block_address as usize;
+    let ib_offset = to_usize(header.index_block_address)?;
     let ib_header_size = 4 + 1 + 1 + os;
     ensure_len(file_data, ib_offset, ib_header_size)?;
 
@@ -463,7 +464,7 @@ pub fn read_extensible_array_chunks(
     let mut pos = ib_offset + ib_header_size;
 
     let mut chunks = Vec::new();
-    let total_elements = header.num_elements as usize;
+    let total_elements = to_usize(header.num_elements)?;
 
     let dmin = header.min_dblk_nelmts as usize;
     if dmin == 0 || !dmin.is_power_of_two() {
@@ -563,7 +564,7 @@ pub fn read_extensible_array_chunks(
             }
             chunks.extend(read_data_block_elements(
                 file_data,
-                addr as usize,
+                to_usize(addr)?,
                 dblk_nelmts,
                 header,
                 offset_size,
@@ -592,7 +593,7 @@ pub fn read_extensible_array_chunks(
         if !is_undefined_addr(sb_addr, offset_size) {
             chunks.extend(read_super_block(
                 file_data,
-                sb_addr as usize,
+                to_usize(sb_addr)?,
                 ndblks,
                 dblk_nelmts,
                 header,
@@ -676,7 +677,7 @@ fn read_super_block(
         if !is_undefined_addr(addr, offset_size) {
             chunks.extend(read_data_block_elements(
                 file_data,
-                addr as usize,
+                to_usize(addr)?,
                 dblk_nelmts,
                 header,
                 offset_size,
diff --git a/crates/clawhdf5-format/src/file_writer.rs b/crates/clawhdf5-format/src/file_writer.rs
index 2568582..1a1af83 100644
--- a/crates/clawhdf5-format/src/file_writer.rs
+++ b/crates/clawhdf5-format/src/file_writer.rs
@@ -3,6 +3,7 @@
 //! Produces valid HDF5 files with v3 superblock, v2 object headers,
 //! link messages, contiguous datasets, inline and dense attributes.
 
+use crate::addr::saturating_usize;
 #[cfg(not(feature = "std"))]
 use alloc::{format, vec, vec::Vec};
 
@@ -336,7 +337,7 @@ pub(crate) fn build_single_block_fractal_heap(
 
     // An object must fit one direct block: the writer has no huge-object
     // path, and libhdf5 cannot read an object that overruns its block.
-    let max_managed = max_direct_block_size as usize - dblock_header_size;
+    let max_managed = saturating_usize(max_direct_block_size) - dblock_header_size;
     if let Some(big) = serialized.iter().find(|s| s.len() > max_managed) {
         return Err(FormatError::SerializationError(format!(
             "a {}-byte message cannot go in dense storage: a fractal heap \
@@ -392,7 +393,7 @@ pub(crate) fn build_single_block_fractal_heap(
     let dblock_addr = frhp_addr + frhp_size as u64;
     let btree_addr = dblock_addr + starting_block_size;
 
-    let data_space = starting_block_size as usize - dblock_header_size;
+    let data_space = saturating_usize(starting_block_size) - dblock_header_size;
     let free_space = data_space - total_data_size;
 
     // Build fractal heap header
@@ -428,7 +429,7 @@ pub(crate) fn build_single_block_fractal_heap(
     debug_assert_eq!(frhp.len(), frhp_size);
 
     // Build direct block: header (with checksum) + data + padding
-    let mut dblock = Vec::with_capacity(starting_block_size as usize);
+    let mut dblock = Vec::with_capacity(saturating_usize(starting_block_size));
     dblock.extend_from_slice(b"FHDB");
     dblock.push(0); // version
     write_offset(&mut dblock, frhp_addr, OFFSET_SIZE);
@@ -446,12 +447,12 @@ pub(crate) fn build_single_block_fractal_heap(
     }
 
     // Pad to full block size
-    dblock.resize(starting_block_size as usize, 0);
+    dblock.resize(saturating_usize(starting_block_size), 0);
 
     // Checksum: computed over entire block with checksum field zeroed
     let dblock_checksum = crate::checksum::jenkins_lookup3(&dblock);
     dblock[cksum_pos..cksum_pos + 4].copy_from_slice(&dblock_checksum.to_le_bytes());
-    debug_assert_eq!(dblock.len(), starting_block_size as usize);
+    debug_assert_eq!(dblock.len(), saturating_usize(starting_block_size));
 
     // Build heap IDs
     let heap_ids: Vec<Vec<u8>> = obj_offsets
@@ -706,7 +707,7 @@ impl HeapIndirectBlock {
                     let cksum_pos = out.len();
                     out.extend_from_slice(&[0u8; 4]); // checksum placeholder
                     out.extend_from_slice(&b.data);
-                    out.resize(d + b.size as usize, 0);
+                    out.resize(d + saturating_usize(b.size), 0);
                     let cksum = crate::checksum::jenkins_lookup3(&out[d..]);
                     out[cksum_pos..cksum_pos + 4].copy_from_slice(&cksum.to_le_bytes());
                     child += b.size;
@@ -740,7 +741,7 @@ impl HeapPacker<'_> {
         nrows: Option<usize>,
     ) -> Result<HeapIndirectBlock, FormatError> {
         let geom = self.geom;
-        let width = geom.width as usize;
+        let width = saturating_usize(geom.width);
         let mut slots = Vec::new();
         let mut off = heap_offset;
         let mut row = 0usize;
@@ -763,7 +764,8 @@ impl HeapPacker<'_> {
                     // A child whose biggest direct block cannot hold the
                     // next object is skipped whole, not walked.
                     let biggest = geom.row_size(child_rows.min(geom.max_direct_rows()) - 1);
-                    if self.objects[self.next].len() > (biggest as usize - geom.dblock_header_size)
+                    if self.objects[self.next].len()
+                        > (saturating_usize(biggest) - geom.dblock_header_size)
                     {
                         slots.push(HeapSlot::Empty);
                         off += size;
@@ -794,7 +796,7 @@ impl HeapPacker<'_> {
     /// objects as fit; leave it unallocated if not even the next one does.
     fn fill_direct(&mut self, heap_offset: u64, size: u64) -> HeapSlot {
         let header = self.geom.dblock_header_size;
-        let capacity = size as usize - header;
+        let capacity = saturating_usize(size) - header;
         let mut data = Vec::new();
         while let Some(obj) = self.objects.get(self.next) {
             if data.len() + obj.len() > capacity {
diff --git a/crates/clawhdf5-format/src/fill_value.rs b/crates/clawhdf5-format/src/fill_value.rs
index 3b18790..d8c34f3 100644
--- a/crates/clawhdf5-format/src/fill_value.rs
+++ b/crates/clawhdf5-format/src/fill_value.rs
@@ -12,6 +12,7 @@
 #[cfg(not(feature = "std"))]
 use alloc::{format, vec, vec::Vec};
 
+use crate::addr::to_usize;
 use crate::chunked_read::{alloc_output, checked_byte_len, list_chunks};
 use crate::data_layout::DataLayout;
 use crate::dataspace::Dataspace;
@@ -256,7 +257,11 @@ pub fn apply_to_unallocated_chunks(
         length_size,
     )?;
     let rank = chunk_dims.len();
-    let ds_dims: Vec<usize> = dataspace.dimensions.iter().map(|&d| d as usize).collect();
+    let ds_dims: Vec<usize> = dataspace
+        .dimensions
+        .iter()
+        .map(|&d| to_usize(d))
+        .collect::<Result<_, _>>()?;
     if rank == 0 || ds_dims.len() != rank || chunk_dims.contains(&0) {
         return Ok(());
     }
@@ -288,7 +293,7 @@ pub fn apply_to_unallocated_chunks(
         let mut cell = 0usize;
         let mut in_range = true;
         for d in 0..rank {
-            let coord = chunk.offsets[d] as usize / chunk_dims[d];
+            let coord = to_usize(chunk.offsets[d])? / chunk_dims[d];
             if coord >= grid[d] {
                 in_range = false;
                 break;
diff --git a/crates/clawhdf5-format/src/filters.rs b/crates/clawhdf5-format/src/filters.rs
index 40d6cd9..672649b 100644
--- a/crates/clawhdf5-format/src/filters.rs
+++ b/crates/clawhdf5-format/src/filters.rs
@@ -3,6 +3,8 @@
 #[cfg(not(feature = "std"))]
 extern crate alloc;
 
+#[cfg(feature = "deflate")]
+use crate::addr::saturating_usize;
 #[cfg(not(feature = "std"))]
 use alloc::{boxed::Box, format, vec, vec::Vec};
 
@@ -1114,7 +1116,11 @@ fn inflate_bounded_into(
     loop {
         let (in_before, out_before) = (inflater.total_in(), inflater.total_out());
         let status = inflater
-            .decompress_vec(&data[in_before as usize..], out, FlushDecompress::Finish)
+            .decompress_vec(
+                &data[saturating_usize(in_before)..],
+                out,
+                FlushDecompress::Finish,
+            )
             .map_err(|e| format!("deflate: {e}"))?;
         if out.len() > limit {
             return Err("deflate: output exceeds size limit".into());
@@ -1132,7 +1138,7 @@ fn inflate_bounded_into(
             }
             Status::Ok | Status::BufError => {
                 // Room left, so the decoder stopped for want of input.
-                if inflater.total_in() as usize >= data.len()
+                if saturating_usize(inflater.total_in()) >= data.len()
                     || (inflater.total_in(), inflater.total_out()) == (in_before, out_before)
                 {
                     return Err("deflate: truncated stream".into());
@@ -1232,7 +1238,11 @@ pub(crate) fn deflate_bounded(data: &[u8], level: u32) -> Result<Vec<u8>, String
     loop {
         let (in_before, out_before) = (deflater.total_in(), deflater.total_out());
         let status = deflater
-            .compress_vec(&data[in_before as usize..], &mut out, FlushCompress::Finish)
+            .compress_vec(
+                &data[saturating_usize(in_before)..],
+                &mut out,
+                FlushCompress::Finish,
+            )
             .map_err(|e| format!("deflate: {e}"))?;
         match status {
             Status::StreamEnd => return Ok(out),
diff --git a/crates/clawhdf5-format/src/filters_blosc2.rs b/crates/clawhdf5-format/src/filters_blosc2.rs
index f4d1690..a609a26 100644
--- a/crates/clawhdf5-format/src/filters_blosc2.rs
+++ b/crates/clawhdf5-format/src/filters_blosc2.rs
@@ -46,6 +46,7 @@
 //! variable-length blocks, dictionaries, lazy chunks, user-defined codecs
 //! and registered filters (e.g. bytedelta), sparse frames.
 
+use crate::addr::saturating_usize;
 use crate::error::FormatError;
 use crate::filter_registry::FilterContext;
 use crate::filters_bitshuffle::bitunshuffle_block;
@@ -546,7 +547,7 @@ fn parse_frame(buf: &[u8], limit: usize) -> Result<Frame<'_>, FormatError> {
         return Err(err("negative size in frame header"));
     }
     let header_len = header_len as usize;
-    let buf = &buf[..frame_len as usize];
+    let buf = &buf[..saturating_usize(frame_len)];
     let cbytes = usize::try_from(cbytes).map_err(|_| err("bad compressed size"))?;
     let data_end = header_len
         .checked_add(cbytes)
diff --git a/crates/clawhdf5-format/src/filters_bzip2.rs b/crates/clawhdf5-format/src/filters_bzip2.rs
index 93e33a3..6989c5b 100644
--- a/crates/clawhdf5-format/src/filters_bzip2.rs
+++ b/crates/clawhdf5-format/src/filters_bzip2.rs
@@ -4,6 +4,7 @@
 //! the compression level). Decoded with the `bzip2` crate's default backend,
 //! `libbz2-rs-sys`, a pure-Rust port of libbzip2.
 
+use crate::addr::saturating_usize;
 use crate::error::FormatError;
 use crate::filter_registry::FilterContext;
 
@@ -28,7 +29,7 @@ pub(crate) fn bzip2_decode(input: &[u8], ctx: &FilterContext<'_>) -> Result<Vec<
     loop {
         let (in_before, out_before) = (dec.total_in(), dec.total_out());
         let status = dec
-            .decompress_vec(&input[in_before as usize..], &mut out)
+            .decompress_vec(&input[saturating_usize(in_before)..], &mut out)
             .map_err(|e| err(&e.to_string()))?;
         if out.len() > limit {
             return Err(err("output exceeds the chunk size"));
@@ -43,7 +44,7 @@ pub(crate) fn bzip2_decode(input: &[u8], ctx: &FilterContext<'_>) -> Result<Vec<
                 .max(1);
             out.try_reserve_exact(grow)
                 .map_err(|_| err("cannot allocate the output buffer"))?;
-        } else if dec.total_in() as usize >= input.len()
+        } else if saturating_usize(dec.total_in()) >= input.len()
             || (dec.total_in(), dec.total_out()) == (in_before, out_before)
         {
             return Err(err("truncated stream"));
@@ -61,7 +62,7 @@ pub(crate) fn bzip2_encode(input: &[u8], ctx: &FilterContext<'_>) -> Result<Vec<
     // bzip2's worst case is about 1% + 600 bytes over the input.
     let mut out = Vec::with_capacity(input.len() + input.len() / 100 + 600);
     loop {
-        let consumed = enc.total_in() as usize;
+        let consumed = saturating_usize(enc.total_in());
         let status = enc
             .compress_vec(&input[consumed..], &mut out, Action::Finish)
             .map_err(|e| cerr(e.to_string()))?;
diff --git a/crates/clawhdf5-format/src/fixed_array.rs b/crates/clawhdf5-format/src/fixed_array.rs
index 8546224..5afc996 100644
--- a/crates/clawhdf5-format/src/fixed_array.rs
+++ b/crates/clawhdf5-format/src/fixed_array.rs
@@ -6,6 +6,7 @@ extern crate alloc;
 #[cfg(not(feature = "std"))]
 use alloc::{format, vec, vec::Vec};
 
+use crate::addr::to_usize;
 use crate::chunk_grid::ChunkGrid;
 use crate::chunked_read::ChunkInfo;
 use crate::error::FormatError;
@@ -158,7 +159,7 @@ pub fn read_fixed_array_chunks(
     offset_size: u8,
     _length_size: u8,
 ) -> Result<Vec<ChunkInfo>, FormatError> {
-    let db_offset = header.data_block_address as usize;
+    let db_offset = to_usize(header.data_block_address)?;
 
     // Parse data block header: FADB(4) + version(1) + client_id(1) + header_address(offset_size)
     let db_header_size = 4 + 1 + 1 + offset_size as usize;
@@ -174,7 +175,7 @@ pub fn read_fixed_array_chunks(
     // Elements start immediately after the data block prefix.
     let elements_start = db_offset + db_header_size;
 
-    let num_elements = header.num_elements as usize;
+    let num_elements = to_usize(header.num_elements)?;
     // A chunk index cannot describe more elements than the file has bytes (each
     // element occupies at least `offset_size` bytes). Reject a corrupt count
     // before it can drive a huge loop or overflow an offset computation.
diff --git a/crates/clawhdf5-format/src/group_v1.rs b/crates/clawhdf5-format/src/group_v1.rs
index a4f97c8..3dff9cd 100644
--- a/crates/clawhdf5-format/src/group_v1.rs
+++ b/crates/clawhdf5-format/src/group_v1.rs
@@ -3,6 +3,7 @@
 #[cfg(not(feature = "std"))]
 use alloc::{string::String, vec::Vec};
 
+use crate::addr::to_usize;
 use crate::btree_v1::collect_symbol_table_nodes;
 use crate::error::FormatError;
 use crate::local_heap::LocalHeap;
@@ -54,7 +55,7 @@ pub(crate) fn v1_group_entries(
     // Parse local heap
     let heap = LocalHeap::parse(
         file_data,
-        sym_table_msg.local_heap_address as usize,
+        to_usize(sym_table_msg.local_heap_address)?,
         offset_size,
         length_size,
     )?;
@@ -70,7 +71,7 @@ pub(crate) fn v1_group_entries(
     let mut entries = Vec::new();
     let mut heap_checked = false;
     for snod_addr in snod_addrs {
-        let snod = SymbolTableNode::parse(file_data, snod_addr as usize, offset_size)?;
+        let snod = SymbolTableNode::parse(file_data, to_usize(snod_addr)?, offset_size)?;
         for entry in &snod.entries {
             // Like libhdf5, look at the heap's free list only once a name is
             // needed: an empty group with a damaged heap still lists.
@@ -152,7 +153,7 @@ fn for_each_v1_soft_link(
 ) -> Result<(), FormatError> {
     let heap = LocalHeap::parse(
         file_data,
-        sym_table_msg.local_heap_address as usize,
+        to_usize(sym_table_msg.local_heap_address)?,
         offset_size,
         length_size,
     )?;
@@ -164,7 +165,7 @@ fn for_each_v1_soft_link(
     )?;
     let mut heap_checked = false;
     for snod_addr in snod_addrs {
-        let snod = SymbolTableNode::parse(file_data, snod_addr as usize, offset_size)?;
+        let snod = SymbolTableNode::parse(file_data, to_usize(snod_addr)?, offset_size)?;
         for entry in &snod.entries {
             if entry.cache_type != CACHE_TYPE_SOFT_LINK {
                 continue;
@@ -242,7 +243,7 @@ pub fn resolve_path(
                 // Not last — must be a group, parse its object header to get symbol table
                 let obj_header = ObjectHeader::parse(
                     file_data,
-                    entry.object_header_address as usize,
+                    to_usize(entry.object_header_address)?,
                     offset_size,
                     length_size,
                 )?;
diff --git a/crates/clawhdf5-format/src/link_message.rs b/crates/clawhdf5-format/src/link_message.rs
index 27044cc..55cb269 100644
--- a/crates/clawhdf5-format/src/link_message.rs
+++ b/crates/clawhdf5-format/src/link_message.rs
@@ -3,6 +3,7 @@
 #[cfg(not(feature = "std"))]
 use alloc::{string::String, vec::Vec};
 
+use crate::addr::to_usize;
 use crate::datatype::CharacterSet;
 use crate::error::FormatError;
 
@@ -247,7 +248,7 @@ impl LinkMessage {
         };
 
         // Link name length
-        let name_len = read_offset(data, pos, name_size_field_width)? as usize;
+        let name_len = to_usize(read_offset(data, pos, name_size_field_width)?)?;
         pos += name_size_field_width as usize;
 
         // Link name
diff --git a/crates/clawhdf5-format/src/local_heap.rs b/crates/clawhdf5-format/src/local_heap.rs
index 39e9b26..b2ef923 100644
--- a/crates/clawhdf5-format/src/local_heap.rs
+++ b/crates/clawhdf5-format/src/local_heap.rs
@@ -3,6 +3,7 @@
 #[cfg(not(feature = "std"))]
 use alloc::string::String;
 
+use crate::addr::to_usize;
 use crate::error::FormatError;
 
 /// Parsed HDF5 Local Heap header.
@@ -140,15 +141,15 @@ impl LocalHeap {
 
     /// Read a null-terminated string from the heap's data segment at the given byte offset.
     pub fn read_string(&self, file_data: &[u8], string_offset: u64) -> Result<String, FormatError> {
-        let seg_addr = self.data_segment_address as usize;
+        let seg_addr = to_usize(self.data_segment_address)?;
         let str_start =
             seg_addr
-                .checked_add(string_offset as usize)
+                .checked_add(to_usize(string_offset)?)
                 .ok_or(FormatError::Overflow(
                     "local heap seg_addr + string_offset overflow".into(),
                 ))?;
         let seg_end = seg_addr
-            .checked_add(self.data_segment_size as usize)
+            .checked_add(to_usize(self.data_segment_size)?)
             .ok_or(FormatError::Overflow(
                 "local heap seg_addr + data_segment_size overflow".into(),
             ))?;
diff --git a/crates/clawhdf5-format/src/object_header.rs b/crates/clawhdf5-format/src/object_header.rs
index 1fc8696..3d7b7f7 100644
--- a/crates/clawhdf5-format/src/object_header.rs
+++ b/crates/clawhdf5-format/src/object_header.rs
@@ -5,6 +5,7 @@ use alloc::vec::Vec;
 
 use byteorder::{ByteOrder, LittleEndian};
 
+use crate::addr::to_usize;
 use crate::error::FormatError;
 use crate::message_type::MessageType;
 
@@ -264,8 +265,8 @@ impl ObjectHeader {
             // Follow continuations (v1 continuation chunks are just raw
             // messages, no signature); check_message has checked the body.
             if msg_type == MessageType::ObjectHeaderContinuation {
-                let cont_offset = read_offset(body, 0, offset_size)? as usize;
-                let cont_length = read_offset(body, offset_size as usize, length_size)? as usize;
+                let cont_offset = to_usize(read_offset(body, 0, offset_size)?)?;
+                let cont_length = to_usize(read_offset(body, offset_size as usize, length_size)?)?;
                 Self::parse_v1_chunk(
                     data,
                     cont_offset,
@@ -339,7 +340,7 @@ impl ObjectHeader {
             _ => unreachable!(),
         };
         ensure_len(data, pos, chunk_size_width as usize)?;
-        let chunk0_size = read_offset(data, pos, chunk_size_width)? as usize;
+        let chunk0_size = to_usize(read_offset(data, pos, chunk_size_width)?)?;
         pos += chunk_size_width as usize;
         // Bit 2: attribute creation order tracked → messages include creation order field
         let has_creation_order = flags & 0x04 != 0;
@@ -472,8 +473,8 @@ impl ObjectHeader {
             let msg_type = MessageType::from_u16(msg_type_raw);
             if msg_type == MessageType::ObjectHeaderContinuation {
                 // check_message has checked the body holds both fields.
-                let cont_off = read_offset(body, 0, offset_size)? as usize;
-                let cont_len = read_offset(body, offset_size as usize, length_size)? as usize;
+                let cont_off = to_usize(read_offset(body, 0, offset_size)?)?;
+                let cont_len = to_usize(read_offset(body, offset_size as usize, length_size)?)?;
                 continuations.push((cont_off, cont_len));
             } else if msg_type == MessageType::Nil {
                 null_count += 1;
diff --git a/crates/clawhdf5-format/src/partial_read.rs b/crates/clawhdf5-format/src/partial_read.rs
index 4c5a01e..ad5dc8e 100644
--- a/crates/clawhdf5-format/src/partial_read.rs
+++ b/crates/clawhdf5-format/src/partial_read.rs
@@ -203,7 +203,12 @@ fn copy_overlap(
     };
     let (src_strides, out_strides) = (strides(src_shape), strides(box_extent));
     let last = rank - 1;
-    let run = ((hi[last] - lo[last]) as usize) * elem_size;
+    // Byte offsets into the in-memory buffers; one that does not fit `usize`
+    // (a 32-bit target) is out of both buffers, like one past their ends.
+    let bytes = |elements: u64| usize::try_from(elements).ok()?.checked_mul(elem_size);
+    let Some(run) = bytes(hi[last] - lo[last]) else {
+        return;
+    };
 
     let mut idx = lo.clone();
     loop {
@@ -213,8 +218,12 @@ fn copy_overlap(
         let out_at: u64 = (0..rank)
             .map(|d| (idx[d] - box_start[d]) * out_strides[d])
             .sum();
-        let (s, o) = (src_at as usize * elem_size, out_at as usize * elem_size);
-        if let (Some(from), Some(to)) = (src.get(s..s + run), out.get_mut(o..o + run)) {
+        if let (Some(s), Some(o)) = (bytes(src_at), bytes(out_at))
+            && let (Some(from), Some(to)) = (
+                src.get(s..s.saturating_add(run)),
+                out.get_mut(o..o.saturating_add(run)),
+            )
+        {
             to.copy_from_slice(from);
         }
         // Advance over every dimension but the last.
diff --git a/crates/clawhdf5-format/src/selection.rs b/crates/clawhdf5-format/src/selection.rs
index de31e46..3dbc6ee 100644
--- a/crates/clawhdf5-format/src/selection.rs
+++ b/crates/clawhdf5-format/src/selection.rs
@@ -19,6 +19,7 @@ use alloc::{vec, vec::Vec};
 
 use core::ops::Range;
 
+use crate::addr::to_usize;
 use crate::error::FormatError;
 
 /// A selection describing which elements of a dataset to access.
@@ -562,7 +563,7 @@ fn decode_hyperslab(r: &mut SelReader, version: u64) -> Result<SerializedSelecti
     if !matches!(enc_size, 2 | 4 | 8) {
         return Err(sel_err("unsupported hyperslab coordinate encoding size"));
     }
-    let rank = r.uint(4)? as usize;
+    let rank = to_usize(r.uint(4)?)?;
     // HDF5 caps dataspace rank at 32 (H5S_MAX_RANK). Reject anything else so a
     // corrupt rank can't drive a huge allocation or read loop.
     if rank == 0 || rank > 32 {
@@ -625,11 +626,11 @@ fn decode_hyperslab(r: &mut SelReader, version: u64) -> Result<SerializedSelecti
         return Err(FormatError::UnexpectedEof {
             expected: r
                 .pos
-                .saturating_add(nblocks.saturating_mul(per_block) as usize),
+                .saturating_add(to_usize(nblocks.saturating_mul(per_block))?),
             available: r.data.len(),
         });
     }
-    let n = nblocks as usize * rank;
+    let n = to_usize(nblocks)? * rank;
     let (mut starts, mut ends) = (Vec::with_capacity(n), Vec::with_capacity(n));
     for _ in 0..nblocks {
         for _ in 0..rank {
@@ -662,7 +663,7 @@ fn blocks_union_coords(
             .filter(|&t| t <= MAX_EXPANDED_POINTS)
             .ok_or_else(|| sel_err("irregular hyperslab selection is too large to expand"))?;
     }
-    let mut out = Vec::with_capacity(total as usize);
+    let mut out = Vec::with_capacity(to_usize(total)?);
     for (s, e) in starts.chunks_exact(rank).zip(ends.chunks_exact(rank)) {
         let mut cur = s.to_vec();
         'block: loop {
diff --git a/crates/clawhdf5-format/src/shared_message.rs b/crates/clawhdf5-format/src/shared_message.rs
index 9667926..5c3227f 100644
--- a/crates/clawhdf5-format/src/shared_message.rs
+++ b/crates/clawhdf5-format/src/shared_message.rs
@@ -23,6 +23,7 @@ use alloc::vec::Vec;
 #[cfg(feature = "std")]
 use std::borrow::Cow;
 
+use crate::addr::to_usize;
 use crate::btree_v2::{BTreeV2Header, collect_btree_v2_records};
 use crate::error::FormatError;
 use crate::fractal_heap::FractalHeapHeader;
@@ -421,7 +422,7 @@ pub fn load_sohm_table(
     else {
         return Ok(None);
     };
-    let ext = ObjectHeader::parse(file_data, ext_addr as usize, offset_size, length_size)?;
+    let ext = ObjectHeader::parse(file_data, to_usize(ext_addr)?, offset_size, length_size)?;
     let Some(msg) = ext
         .messages
         .iter()
@@ -432,7 +433,7 @@ pub fn load_sohm_table(
     let table_msg = parse_sohm_table_message(&msg.data, offset_size)?;
     parse_sohm_table(
         file_data,
-        table_msg.table_address as usize,
+        to_usize(table_msg.table_address)?,
         table_msg.nindexes,
         offset_size,
     )
@@ -505,7 +506,7 @@ pub fn resolve_sohm_message(
 
     let fh_header = FractalHeapHeader::parse(
         file_data,
-        index.heap_addr as usize,
+        to_usize(index.heap_addr)?,
         offset_size,
         length_size,
     )?;
@@ -587,7 +588,7 @@ pub fn resolve_shared_message_with_sohm(
     ) {
         (Some(addr), _) => {
             let target_header =
-                ObjectHeader::parse(file_data, addr as usize, offset_size, length_size)?;
+                ObjectHeader::parse(file_data, to_usize(addr)?, offset_size, length_size)?;
             for msg in &target_header.messages {
                 if msg.msg_type == target_msg_type && !is_shared(msg.flags) {
                     return Ok(msg.data.clone());
diff --git a/crates/clawhdf5-format/src/vds.rs b/crates/clawhdf5-format/src/vds.rs
index d67b832..df1b068 100644
--- a/crates/clawhdf5-format/src/vds.rs
+++ b/crates/clawhdf5-format/src/vds.rs
@@ -15,6 +15,7 @@
 #[cfg(not(feature = "std"))]
 use alloc::{format, string::String, vec, vec::Vec};
 
+use crate::addr::to_usize;
 use crate::data_layout::{DataLayout, VdsMapping, parse_vds_mappings};
 use crate::dataspace::Dataspace;
 use crate::datatype::Datatype;
@@ -208,7 +209,7 @@ fn load_mappings(
         return Ok(Vec::new());
     };
     let coll =
-        crate::global_heap::GlobalHeapCollection::parse(file_data, addr as usize, length_size)?;
+        crate::global_heap::GlobalHeapCollection::parse(file_data, to_usize(addr)?, length_size)?;
     let index = u16::try_from(*global_heap_index)
         .map_err(|_| vds_err("VDS mapping heap index out of range"))?;
     let obj = coll
@@ -611,12 +612,12 @@ fn scatter(
         return Err(vds_err("virtual/source selection element counts differ"));
     }
     for (&v, &s) in vidx.iter().zip(sidx) {
-        let (vo, so) = (v as usize * elem_size, s as usize * elem_size);
+        let (vo, so) = (to_usize(v)? * elem_size, to_usize(s)? * elem_size);
         if vo + elem_size > out.len() || so + elem_size > src.len() {
             return Err(vds_err("virtual dataset selection out of bounds"));
         }
         out[vo..vo + elem_size].copy_from_slice(&src[so..so + elem_size]);
-        mapped[v as usize] = true;
+        mapped[to_usize(v)?] = true;
     }
     Ok(())
 }
@@ -747,7 +748,7 @@ fn selection_indices(
                     return Err(vds_err("VDS selection blocks overlap"));
                 }
             }
-            let mut out = Vec::with_capacity(volume as usize);
+            let mut out = Vec::with_capacity(to_usize(volume)?);
             for (s, e) in starts.chunks_exact(rank).zip(ends.chunks_exact(rank)) {
                 let mut cur = s.to_vec();
                 'block: loop {
@@ -868,7 +869,7 @@ fn load_source_file(whole: &mut [u8]) -> Result<(), FormatError> {
     // read as before, up to its length.
     let end = sb
         .data_end(base as u64, whole.len() as u64)
-        .map_or(whole.len(), |e| base + e as usize);
+        .map_or(Ok(whole.len()), |e| to_usize(e).map(|e| base + e))?;
     crate::superblock_ext::apply_cache_image_in_place(&mut whole[base..end], &sb)
 }
 
@@ -921,7 +922,7 @@ fn open_source(file_data: &[u8], path: &str) -> Result<Option<OpenSource>, Forma
         Err(FormatError::PathNotFound(_)) => return Ok(None),
         Err(e) => return Err(e),
     };
-    let header = crate::object_header::ObjectHeader::parse(file_data, addr as usize, os, ls)?;
+    let header = crate::object_header::ObjectHeader::parse(file_data, to_usize(addr)?, os, ls)?;
     let mut src = OpenSource {
         offset_size: os,
         length_size: ls,
diff --git a/crates/clawhdf5-format/src/vl_data.rs b/crates/clawhdf5-format/src/vl_data.rs
index 3a54c1a..a998a62 100644
--- a/crates/clawhdf5-format/src/vl_data.rs
+++ b/crates/clawhdf5-format/src/vl_data.rs
@@ -9,6 +9,7 @@ use alloc::{collections::BTreeMap, format, string::String, vec, vec::Vec};
 #[cfg(feature = "std")]
 use std::collections::BTreeMap;
 
+use crate::addr::to_usize;
 use crate::error::FormatError;
 use crate::global_heap::{GlobalHeapCollection, GlobalHeapIndex};
 
@@ -55,7 +56,7 @@ pub fn parse_vl_references(
 ) -> Result<Vec<VlElement>, FormatError> {
     let elem_size = 4 + offset_size as usize + 4; // length + address + index
     let total =
-        (num_elements as usize)
+        to_usize(num_elements)?
             .checked_mul(elem_size)
             .ok_or(FormatError::UnexpectedEof {
                 expected: usize::MAX,
@@ -68,7 +69,7 @@ pub fn parse_vl_references(
         });
     }
 
-    let mut elements = Vec::with_capacity(num_elements as usize);
+    let mut elements = Vec::with_capacity(to_usize(num_elements)?);
     let mut pos = 0;
 
     for _ in 0..num_elements {
@@ -406,7 +407,7 @@ impl<'a> VlResolver<'a> {
             let index =
                 GlobalHeapCollection::parse_index(self.file_data, offset, self.length_size)?;
             // parse_index checked that the collection lies in the file.
-            let end = offset + index.collection_size as usize;
+            let end = offset + to_usize(index.collection_size)?;
             self.check_overlap(offset, end)?;
             let coll = CachedCollection::new(index);
             if self.cached_bytes.saturating_add(coll.cost()) > self.budget {
diff --git a/scripts/check-32bit-casts.sh b/scripts/check-32bit-casts.sh
new file mode 100755
index 0000000..cfed6f5
--- /dev/null
+++ b/scripts/check-32bit-casts.sh
@@ -0,0 +1,35 @@
+#!/usr/bin/env bash
+# CI check: clawhdf5-format has no truncating `u64 as usize` cast on a 32-bit
+# target. HDF5 addresses and lengths are 64-bit; on wasm32 (or any 32-bit
+# target) such a cast silently wraps an address past 4 GiB onto another part
+# of the file. File values go through `addr::to_usize` (a clean error) and
+# in-memory counts through `addr::saturating_usize`.
+#
+# Lints wasm32-unknown-unknown with clippy's cast_possible_truncation and
+# fails on any u64 -> usize finding (other truncations are not checked here).
+#
+# Usage:
+#   ./scripts/check-32bit-casts.sh
+#
+# Prerequisites:
+#   rustup target add wasm32-unknown-unknown
+
+set -euo pipefail
+
+TARGET="wasm32-unknown-unknown"
+echo "==> Checking for truncating u64 -> usize casts in clawhdf5-format ($TARGET)"
+
+out=$(cargo clippy -p clawhdf5-format --target "$TARGET" \
+    --features plugin-filters --message-format short \
+    -- -A clippy::all -W clippy::cast_possible_truncation 2>&1) || {
+    echo "$out"
+    echo "==> clippy failed" >&2
+    exit 1
+}
+found=$(grep -F 'casting `u64` to `usize`' <<<"$out" || true)
+if [ -n "$found" ]; then
+    echo "$found"
+    echo "==> use addr::to_usize (file values) or addr::saturating_usize (in-memory counts)" >&2
+    exit 1
+fi
+echo "==> no truncating u64 -> usize casts"
diff --git a/scripts/ci-test.sh b/scripts/ci-test.sh
index 3012cba..c7c77dd 100755
--- a/scripts/ci-test.sh
+++ b/scripts/ci-test.sh
@@ -147,6 +147,8 @@ run_step "wasm32 clippy (clawhdf5-wasm)" cargo clippy \
     --target wasm32-unknown-unknown \
     --all-targets \
     -- -D warnings
+# A 64-bit file address must not wrap on a 32-bit target.
+run_step "check-32bit-casts.sh" "$SCRIPT_DIR/check-32bit-casts.sh"
 
 # The built wasm package, run under Node against h5py/netCDF4-written files,
 # and the viewer page in headless Chromium when one is found.