diff --git a/README.md b/README.md
index ab64be4..125f821 100644
--- a/README.md
+++ b/README.md
@@ -229,7 +229,12 @@ const { value, profiling } = await fn()
 console.log(profiling)
 // {
 //   cpu: 42.5,
-//   memory: 128204800,
+//   memory: {
+//     total: 128204800,
+//     used: 85198848,
+//     heap: 2831776,
+//     external: 83961856
+//   },
 //   phases: {
 //     compile: 0,
 //     spawn: 48,
@@ -242,7 +247,11 @@ console.log(profiling)
 Each execution includes profiling data:
 
 - **cpu** — CPU time (user + system) consumed by the process, in milliseconds.
-- **memory** — Peak RSS (Resident Set Size) of the process, in bytes.
+- **memory** — Memory breakdown, in bytes, sampled when the function returns. No single number describes the memory a function used, so each field answers a different question:
+  - **total** — Resident set size of the whole isolate process. Includes the ~43MB Node.js runtime baseline, so it is never zero.
+  - **used** — `total` minus the baseline measured before the function ran. This is the part attributable to the function.
+  - **heap** — V8 heap in use. This is the only field bounded by the [`memory`](#memory) limit.
+  - **external** — Off-heap memory (`Buffer`, `ArrayBuffer`, typed arrays). **Not** bounded by the [`memory`](#memory) limit, and invisible to `total` until the pages are written to.
 - **phases** — Wall-clock time breakdown of each execution stage, in milliseconds:
   - **compile** — Time waiting for code compilation (dependency detection, package install, esbuild bundling). This is `0` after the first call since the result is cached.
   - **spawn** — Process creation, Node.js boot, and template setup overhead.
@@ -256,20 +265,16 @@ You can limit a **isolated-function** with memory:
 ```js
 const fn = isolatedFunction(() => {
   const storage = []
-  const oneMegabyte = 1024 * 1024
-  while (storage.length < 78) {
-    const array = new Uint8Array(oneMegabyte)
-    for (let ii = 0; ii < oneMegabyte; ii += 4096) {
-      array[ii] = 1
-    }
-    storage.push(array)
-  }
+  while (true) storage.push('x'.repeat(1024))
 }, { memory: 64 })
 
 await fn()
 // =>  MemoryError: Out of memory
 ```
 
+> [!IMPORTANT]
+> The `memory` limit bounds the **V8 heap** (`profiling.memory.heap`), which is where ordinary objects and strings live. It does **not** bound off-heap memory (`profiling.memory.external`): `Buffer`, `ArrayBuffer`, and typed arrays are allocated outside the heap and can grow past the limit without raising `MemoryError`. If a function must be held to a total memory budget, enforce it outside the process (for example a cgroup) rather than relying on `memory` alone.
+
 or by execution time:
 
 ```js
diff --git a/src/index.d.ts b/src/index.d.ts
index c058edf..6594df2 100644
--- a/src/index.d.ts
+++ b/src/index.d.ts
@@ -12,11 +12,22 @@ export interface Phases {
   total: number
 }
 
+export interface Memory {
+  /** Resident set size of the whole isolate process, in bytes */
+  total: number
+  /** Resident memory attributable to the function (total minus the pre-execution baseline), in bytes */
+  used: number
+  /** V8 heap in use, in bytes. This is the only figure bounded by the `memory` limit */
+  heap: number
+  /** Off-heap memory (Buffer, ArrayBuffer, TypedArray), in bytes. Not bounded by the `memory` limit */
+  external: number
+}
+
 export interface Profiling {
   /** CPU time (user + system) in milliseconds */
   cpu: number
-  /** Memory usage in bytes */
-  memory: number
+  /** Memory usage breakdown, in bytes */
+  memory: Memory
   /** Bundled code size in bytes */
   size: number
   /** Execution phase durations in milliseconds */
diff --git a/src/index.js b/src/index.js
index cda0d03..aa9fdbe 100644
--- a/src/index.js
+++ b/src/index.js
@@ -16,6 +16,12 @@ const createError = ({ name, message, ...props }) => {
   return error
 }
 
+/* V8 aborts (SIGABRT) on heap exhaustion for any realistic --max-old-space-size,
+   and only traps (SIGTRAP) when the limit is too small to boot the heap. Gate the
+   abort on V8's message so an unrelated abort is not reported as a memory error. */
+const isOutOfMemory = ({ signalCode, stderr }) =>
+  signalCode === 'SIGTRAP' || (signalCode === 'SIGABRT' && /out of memory/i.test(stderr ?? ''))
+
 const [nodeMajor] = process.version.slice(1).split('.').map(Number)
 
 const PERMISSION_FLAG = nodeMajor >= 24 ? '--permission' : '--experimental-permission'
@@ -94,7 +100,7 @@ module.exports = ({ tmpdir, nodePaths } = {}) => {
         debug.error(serializeError(error))
         const profiling = { phases: { total: total() } }
 
-        if (error.signalCode === 'SIGTRAP') {
+        if (isOutOfMemory(error)) {
           throw createError({
             name: 'MemoryError',
             message: 'Out of memory',
diff --git a/src/template/index.js b/src/template/index.js
index 30aa122..e7ed159 100644
--- a/src/template/index.js
+++ b/src/template/index.js
@@ -4,7 +4,9 @@ const SERIALIZE_ERROR = require('./serialize-error')
 
 module.exports = snippet => `;(send => {
   process.stdout.write = function () {}
-  const respond = (isFulfilled, value, run, logs = {}) => { const {user, system} = process.cpuUsage(); send(JSON.stringify({isFulfilled, logging: logs, value, profiling: {cpu: (user + system) / 1000, memory: process.memoryUsage().rss, run}})) }
+  const baseline = process.memoryUsage().rss
+  const memory = () => { const m = process.memoryUsage(); return {total: m.rss, used: Math.max(0, m.rss - baseline), heap: m.heapUsed, external: m.external} }
+  const respond = (isFulfilled, value, run, logs = {}) => { const {user, system} = process.cpuUsage(); send(JSON.stringify({isFulfilled, logging: logs, value, profiling: {cpu: (user + system) / 1000, memory: memory(), run}})) }
 
   return Promise.resolve().then(async () => {
     const args = JSON.parse(process.argv[2])
diff --git a/test/error.js b/test/error.js
index c3554b9..74a8c24 100644
--- a/test/error.js
+++ b/test/error.js
@@ -111,6 +111,34 @@ test('handle OOM', async t => {
   t.is(typeof error.profiling.phases.total, 'number')
 })
 
+/* `memory: 1` is the only limit small enough that V8 traps instead of aborting.
+   Every production-sized limit exhausts the heap through the normal OOM path,
+   which exits with SIGABRT. Growing the heap (not off-heap buffers) is what the
+   limit actually bounds, so this is the shape of a real customer OOM. */
+test('handle OOM at a production-sized limit', async t => {
+  const fn = isolatedFunction(
+    () => {
+      const storage = []
+      while (true) storage.push('x'.repeat(1024))
+    },
+    { memory: 16 }
+  )
+
+  const error = await t.throwsAsync(fn())
+
+  t.is(error.name, 'MemoryError')
+  t.is(error.message, 'Out of memory')
+  t.is(typeof error.profiling.phases.total, 'number')
+})
+
+test('an abort that is not OOM is not reported as MemoryError', async t => {
+  const fn = isolatedFunction(() => process.abort(), { memory: 16 })
+
+  const error = await t.throwsAsync(fn())
+
+  t.not(error.name, 'MemoryError')
+})
+
 test('handle filesystem permissions', async t => {
   {
     const fn = isolatedFunction(() => {
diff --git a/test/index.js b/test/index.js
index da3facf..38acb59 100644
--- a/test/index.js
+++ b/test/index.js
@@ -156,7 +156,18 @@ test('memory profiling', async t => {
 
   t.is(value, undefined)
   t.is(typeof profiling.cpu, 'number')
-  t.is(typeof profiling.memory, 'number')
+  t.is(typeof profiling.memory.total, 'number')
+  t.is(typeof profiling.memory.used, 'number')
+  t.is(typeof profiling.memory.heap, 'number')
+  t.is(typeof profiling.memory.external, 'number')
+
+  // the 78MB of touched Uint8Array is off-heap, so it lands in external and
+  // shows up as memory the function is accountable for, not as heap
+  t.true(profiling.memory.external > 64 * 1024 * 1024)
+  t.true(profiling.memory.used > 64 * 1024 * 1024)
+  t.true(profiling.memory.total >= profiling.memory.used)
+  t.true(profiling.memory.heap < profiling.memory.external)
+
   t.is(typeof profiling.phases.install, 'number')
   t.is(typeof profiling.phases.build, 'number')
   t.is(typeof profiling.phases.spawn, 'number')
diff --git a/test/index.test-d.ts b/test/index.test-d.ts
index fcd0be2..8a6a5d3 100644
--- a/test/index.test-d.ts
+++ b/test/index.test-d.ts
@@ -4,6 +4,7 @@ import createIsolatedFunction, {
   FailureResult,
   SuccessResult,
   Profiling,
+  Memory,
   Logging,
   IsolatedFunctionOptions,
   IsolatedFunctionInstance,
@@ -99,7 +100,11 @@ if (execResult.isFulfilled) {
   const { profiling, logging } = execResult
 
   expectType<number>(profiling.cpu)
-  expectType<number>(profiling.memory)
+  expectType<Memory>(profiling.memory)
+  expectType<number>(profiling.memory.total)
+  expectType<number>(profiling.memory.used)
+  expectType<number>(profiling.memory.heap)
+  expectType<number>(profiling.memory.external)
   expectType<number>(profiling.phases.compile)
   expectType<number>(profiling.phases.spawn)
   expectType<number>(profiling.phases.run)