diff --git a/src/engine.ts b/src/engine.ts
index c9b4d2e4..9446cc37 100644
--- a/src/engine.ts
+++ b/src/engine.ts
@@ -563,7 +563,8 @@ function dynamicReplacement(_file_rr, _config_rr, _data_rr) {
 	}
 
 	if (needsByteOrSize && -1 === _bytes) {
-		_bytes = Buffer.from(_text).length;
+		// The size in the encoding of the file, not of the text as utf8
+		_bytes = Buffer.byteLength(_text, _config_rr.encoding);
 		_size = readableSize(_bytes);
 	}
 
diff --git a/test/js/encoding.test.js b/test/js/encoding.test.js
index 8fc75d70..ae651cfc 100644
--- a/test/js/encoding.test.js
+++ b/test/js/encoding.test.js
@@ -125,18 +125,18 @@ describe('--encoding', () => {
 			expect((await rr('æ', 'text.length + text', config)).out).toBe('f3fæoo\n');
 		});
 
-		test.todo(
-			'BUG #688: with --replacement-js `bytes` is the size of the file in its encoding',
-			async () => {
-				fs.writeFileSync('my.file', latin1());
-				const config = {
-					includeGlob: ['my.file'],
-					encoding: 'latin1',
-					replacementJs: true,
-					output: true,
-				};
-				expect((await rr('^', 'bytes + ":"', config)).out).toBe('3:fæo');
-			}
-		);
+		test('with --replacement-js `bytes` and `size` are the size of the file in its encoding (#688)', async () => {
+			// 1500 times `æ`: one byte each in latin1, but two in utf8 (that would give 3000 and 2.9 KB)
+			fs.writeFileSync('my.file', Buffer.alloc(1500, 0xe6));
+			const config = {
+				includeGlob: ['my.file'],
+				encoding: 'latin1',
+				replacementJs: true,
+				output: true,
+			};
+			expect((await rr('^', 'bytes + ":" + size + ":"', config)).out).toBe(
+				'1500:1.5 KB:' + 'æ'.repeat(1500) + '\n'
+			);
+		});
 	});
 });