MP3 to M4B Audiobook Maker

Three steps

1. Drop all the MP3s for one book. They're sorted by file name, so Track 2 comes before Track 10.
2. Check the chapter list. Use the arrows to fix the order and type over any chapter name.
3. Hit Make M4B audiobook. The first run downloads the audio engine (31 MB, one time). After that, encoding runs on your device.

Why M4B instead of a folder of MP3s

A folder of 40 MP3s works, but every player treats it as 40 songs. Shuffle mixes up the chapters. Your place is saved per track, not per book. The playback speed setting often resets between files. M4B fixes all of that. It's a single MP4 audio file flagged as an audiobook, with a chapter table inside it. Audiobook players remember your position to the second, show a chapter list you can jump around in, and keep your speed setting. On an iPhone, an M4B added through the Files app or synced from a Mac shows up in Apple Books under Audiobooks, not in Music.

What this tool does to your audio

Every file is decoded, resampled to a common format, and joined end to end. The result is encoded once as AAC, the codec M4B players expect. Each input file becomes one chapter. The chapter starts exactly where that file started in the joined audio, and its name comes from the file name, minus leading track numbers like 01 -. The title, author, and cover go into the file's metadata, so players show the right artwork and book name.

The spoken word setting (64 kbps mono) is what Audible and most audiobook stores use for narration. It sounds the same as a 128 kbps stereo MP3 of a single voice, at half the size. A 10-hour book comes out around 290 MB. Pick music or drama for full-cast productions, sound effects, or anything where stereo matters.

How long it takes

Encoding runs on one CPU core inside the browser. On a recent laptop, expect roughly 30 to 60 times real time. A 1-hour book takes 1 to 2 minutes, and a 10-hour book takes 10 to 20 minutes. Phones are slower. Keep the tab in the foreground on a phone, because mobile browsers pause background tabs. The progress bar shows how much of the book has been encoded.

Why it runs in your browser

Audiobook files are big, often several hundred megabytes. Upload-based converters make you wait for the upload, cap the file size, or charge for anything over an hour. Here the files are read straight from your disk into this tab, so a 500 MB book is ready to encode the moment you drop it in. It's also private. Recordings of your own voice, a family member reading, or an unreleased narration never touch a server. You can check this: load the page, turn off Wi-Fi after the audio engine has downloaded, and the conversion still works.

Common problems and fixes

• Chapters are in the wrong order. Files are sorted naturally by name. If the names don't carry the order (like intro.mp3, part-a.mp3), use the arrows to reorder before converting.
• The tab crashed on a very long book. The whole book is held in memory while encoding. Most laptops handle 1 GB of input without trouble; phones start failing at a few hundred MB. Split the book in two and make two M4Bs, or use a desktop browser.
• Chapter marks drift by a second or two. Some MP3s have no length header, so their duration is estimated from the bitrate. Re-exporting those files from any audio editor fixes it.
• The cover doesn't show. Use a square JPG or PNG. WEBP and HEIC can't be embedded in M4B. A 600×600 to 1400×1400 image is ideal.
• Apple Books files it under Music. Add it through the Books app (Share → Books) or the Files app, not the Music app. The file is marked as an audiobook, but the Music app ignores that flag.

Frequently asked questions