diff --git a/test/parallel/test-stream-writable-to-web-strategy-size.js b/test/parallel/test-stream-writable-to-web-strategy-size.js
new file mode 100644
index 000000000000..d7914ebb8ecb
--- /dev/null
+++ b/test/parallel/test-stream-writable-to-web-strategy-size.js
@@ -0,0 +1,34 @@
+'use strict';
+require('../common');
+
+// Verifies that Writable.toWeb() sizes queued chunks by their byte length for a
+// non-object-mode stream, instead of counting each chunk as one, and that an
+// object-mode stream still counts chunks.
+
+const assert = require('node:assert');
+const { Writable } = require('node:stream');
+const { test } = require('node:test');
+
+function writerFor(objectMode) {
+  // The write callback is never invoked, so chunks stay queued.
+  const writable = new Writable({ highWaterMark: 100, objectMode, write() {} });
+  return Writable.toWeb(writable).getWriter();
+}
+
+test('non-object mode sizes chunks by byte length', () => {
+  const writer = writerFor(false);
+
+  assert.strictEqual(writer.desiredSize, 100);
+  writer.write(new Uint8Array(3));
+  assert.strictEqual(writer.desiredSize, 97);
+  writer.write(Buffer.alloc(7));
+  assert.strictEqual(writer.desiredSize, 90);
+});
+
+test('object mode counts chunks', () => {
+  const writer = writerFor(true);
+
+  assert.strictEqual(writer.desiredSize, 100);
+  writer.write(new Uint8Array(3));
+  assert.strictEqual(writer.desiredSize, 99);
+});