M4B to MP3: Split an Audiobook by Chapter

Three steps

1. Drop the .m4b. The chapter list appears so you can check it before converting. The first time, this downloads the audio engine (31 MB, once).
2. Choose one MP3 per chapter, one single MP3, or M4A per chapter.
3. Hit Convert. Per-chapter files arrive as a zip, named 01 - Chapter Title.mp3, 02 - ….

Which output to pick

• One MP3 per chapter is the most compatible. Car stereos, USB sticks, older iPods, kids' audio players, and plain music apps all play MP3s in file-name order. Each file is tagged with the chapter title, book title, author, and track number.
• One single MP3 suits podcast apps and players that remember your position inside one long file. The chapter marks are written as ID3 chapter frames, which some apps (Pocket Casts, Overcast, VLC) show as a chapter list.
• M4A per chapter skips re-encoding entirely. It cuts the existing audio at chapter boundaries, so it finishes in seconds and loses no quality. Use it when your player handles AAC/M4A but not audiobook chapters, as many Android music apps don't.

How the split works

An M4B keeps its chapters as a table of start and end times next to the audio. This tool reads that table, then cuts the audio at each boundary and encodes each piece to MP3 with LAME, the same encoder behind most MP3s you already own. If the book has no chapter table, you get one MP3 of the whole thing instead. Spoken word at 64 kbps mono is about 29 MB per hour and sounds the same as the source for a single narrator.

How long it takes

MP3 encoding runs on one CPU core in the browser, at about 20 to 50 times real time on a recent laptop. A 10-hour book takes somewhere between 12 and 30 minutes, with the progress bar tracking each chapter. The M4A option doesn't re-encode, so it runs about as fast as your disk can copy. On a phone, keep the tab open and in the foreground until it finishes.

Your audiobook never leaves your device

Upload-based converters mean pushing a 300 MB file up, waiting in a queue, then downloading it again. Most also cap free conversions at around 100 MB, which is less than one audiobook. Here the file is read straight from your disk into this browser tab and processed locally. Once the audio engine has loaded, you can turn off Wi-Fi and the conversion still completes.

Common problems and fixes

• "DRM-protected" or the file won't open. Audible .aax/.aaxc files and Apple Books purchases are encrypted, and this tool doesn't remove DRM. DRM-free audiobooks (Libro.fm, Downpour, LibriVox, your own rips) all work.
• Only one file came out. The book has no chapter table. Some rippers create M4Bs without chapters. Try the M4A option to confirm, or use a player that shows chapters to check the source.
• Chapter names are "Chapter 1, Chapter 2…". That's what the M4B stores. The tool uses the names embedded in the file.
• The tab crashed. Very long books (20+ hours) can exceed a phone's memory. Use a laptop, or choose M4A per chapter, which uses far less memory than encoding.

Frequently asked questions