diff --git a/README.md b/README.md
index 65f2e56..927e54b 100644
--- a/README.md
+++ b/README.md
@@ -213,6 +213,19 @@ Names begin with an ASCII letter or underscore. Array lengths are decimal values
 
 The `lp64` ABI uses 8-byte pointers; `ilp32` uses 4-byte pointers. Both ABIs align 64-bit integers and floats to 8 bytes under `lp64`; under `ilp32` they keep their 8-byte size but use 4-byte alignment. Each field starts at an offset aligned to its type, and the final struct size is rounded up to the struct alignment. Padding is the space between fields and at the end.
 
+The pointer-width integers `usize` and `isize` have the size and alignment of `ptr`: 8 and 8 under `lp64`, 4 and 4 under `ilp32`. They are the types to use for lengths and offsets that follow the platform, such as `size_t` in C or `usize` in Rust. For `struct Buf { u8 tag; usize len; isize off; u16 flags; }`, `structpad layout --abi ilp32` prints:
+
+```text
+struct Buf: size 16, align 4, padding 5
+offset   size  field
+     0      1  tag: u8
+     1      3  (padding)
+     4      4  len: usize
+     8      4  off: isize
+    12      2  flags: u16
+    14      2  (padding)
+```
+
 ## Exit codes
 
 - `0`: command succeeded, or `check` found no struct over the padding limit.
diff --git a/src/structpad/types.py b/src/structpad/types.py
index 4748138..9ecbc0c 100644
--- a/src/structpad/types.py
+++ b/src/structpad/types.py
@@ -46,6 +46,8 @@ def alignment(self, abi: str) -> int:
         ("i64", 8, 8, 8, 4),
         ("f64", 8, 8, 8, 4),
         ("ptr", 8, 8, 4, 4),
+        ("usize", 8, 8, 4, 4),
+        ("isize", 8, 8, 4, 4),
     )
 }
 
diff --git a/tests/test_pointer_ints.py b/tests/test_pointer_ints.py
new file mode 100644
index 0000000..742579c
--- /dev/null
+++ b/tests/test_pointer_ints.py
@@ -0,0 +1,115 @@
+"""Tests for pointer-width integer primitives."""
+
+import importlib
+import json
+import sys
+from io import BytesIO
+from pathlib import Path
+
+_SOURCE_DIRECTORY = str(Path(__file__).resolve().parents[1] / "src")
+if _SOURCE_DIRECTORY not in sys.path:
+    sys.path.insert(0, _SOURCE_DIRECTORY)
+
+from structpad.types import get_primitive
+
+run = importlib.import_module("structpad.cli").run
+
+_BUF = b"struct Buf {\n    u8 tag;\n    usize len;\n    isize off;\n    u16 flags;\n}\n"
+
+
+def _run(args: list[str], source: bytes = _BUF) -> tuple[int, bytes, bytes]:
+    stdin = BytesIO(source)
+    stdout = BytesIO()
+    stderr = BytesIO()
+    status = run(args, stdin, stdout, stderr)
+    return status, stdout.getvalue(), stderr.getvalue()
+
+
+def test_usize_and_isize_have_the_size_and_alignment_of_ptr() -> None:
+    pointer = get_primitive("ptr")
+    assert pointer is not None
+
+    for name in ("usize", "isize"):
+        primitive = get_primitive(name)
+        assert primitive is not None
+        for abi, expected in (("lp64", (8, 8)), ("ilp32", (4, 4))):
+            expected_size, expected_alignment = expected
+            assert primitive.size(abi) == expected_size
+            assert primitive.alignment(abi) == expected_alignment
+            assert primitive.size(abi) == pointer.size(abi)
+            assert primitive.alignment(abi) == pointer.alignment(abi)
+
+
+def test_layout_under_lp64() -> None:
+    status, stdout, stderr = _run(["layout", "-"])
+
+    assert status == 0
+    assert stdout == (
+        b"struct Buf: size 32, align 8, padding 13\n"
+        b"offset   size  field\n"
+        b"     0      1  tag: u8\n"
+        b"     1      7  (padding)\n"
+        b"     8      8  len: usize\n"
+        b"    16      8  off: isize\n"
+        b"    24      2  flags: u16\n"
+        b"    26      6  (padding)\n"
+    )
+    assert stderr == b""
+
+
+def test_layout_under_ilp32() -> None:
+    status, stdout, stderr = _run(["layout", "--abi", "ilp32", "-"])
+
+    assert status == 0
+    assert stdout == (
+        b"struct Buf: size 16, align 4, padding 5\n"
+        b"offset   size  field\n"
+        b"     0      1  tag: u8\n"
+        b"     1      3  (padding)\n"
+        b"     4      4  len: usize\n"
+        b"     8      4  off: isize\n"
+        b"    12      2  flags: u16\n"
+        b"    14      2  (padding)\n"
+    )
+    assert stderr == b""
+
+
+def test_suggest_and_check_use_the_pointer_width() -> None:
+    status, stdout, stderr = _run(["suggest", "-"])
+
+    assert status == 0
+    assert stdout == (
+        b"struct Buf: size 32 -> 24 (saves 8)\n"
+        b"  usize len;\n"
+        b"  isize off;\n"
+        b"  u16 flags;\n"
+        b"  u8 tag;\n"
+    )
+    assert stderr == b""
+
+    status, stdout, stderr = _run(["check", "--max-padding", "8", "-"])
+
+    assert status == 1
+    assert stdout == (
+        b"Buf: 13 bytes of padding (limit 8)\n1 of 1 structs over the padding limit\n"
+    )
+    assert stderr == b""
+
+
+def test_json_layout_names_the_type_and_other_names_stay_unknown() -> None:
+    status, stdout, stderr = _run(["layout", "--format", "json", "-"])
+
+    assert status == 0
+    layout = json.loads(stdout)
+    len_field = next(field for field in layout[0]["fields"] if field["name"] == "len")
+    assert len_field["type"] == "usize"
+    assert len_field["offset"] == 8
+    assert len_field["size"] == 8
+    assert stderr == b""
+
+    invalid_source = b"struct Buf {\n    size_t n;\n}\n"
+    status, stdout, stderr = _run(["layout", "-"], invalid_source)
+
+    assert status == 2
+    assert stdout == b""
+    assert stderr == b'structpad: <stdin>:2: unknown type "size_t"\n'