M4B to MP3 Converter (Audiobook, Split by Chapter)

M4B is the audiobook format of Apple Books, iTunes and many audiobook stores: AAC audio in an MP4 file with chapter marks, a cover and "remember my place" support. Many MP3 players, car stereos and older phones cannot play it. This converter turns a DRM-free M4B into MP3 in your browser, either as one MP3 that keeps the chapters as ID3 chapter marks, as one MP3 per chapter named after the chapter titles, or in parts of a set length.

How to use this tool

1. Choose an M4B (or M4A) audiobook.
2. Pick "One MP3 per chapter", "One MP3 with the chapter marks kept" or "Parts of a set length".
3. Keep 64 kbps mono for clear speech in small files; choose more for music or radio plays.
4. Press Calculate and download the files (several files come as one ZIP).

Formula

MP3 size ≈ bitrate (kbps) × 1000 ÷ 8 × length (s)

The ffmpeg engine (about 30 MB, downloaded once) reads the chapter list (start, end and title of every chapter) and encodes the audio with LAME. Split files are cut at the exact chapter times and tagged with the chapter title, track number, book title, author and cover. The single-file option writes the chapters as ID3v2 CHAP frames, which podcast and audiobook apps show; many simple MP3 players ignore them. Copy-protected audiobooks (Audible AAX, Apple Books purchases with DRM) cannot be converted by any honest tool.

Example

A 10-hour audiobook at 64 kbps mono becomes 64,000 ÷ 8 × 36,000 = 288 MB of MP3; at 32 kbps it is 144 MB and still clear for speech. Split by chapter, a book with 24 chapters gives 24 files named "01 - Chapter One.mp3" and so on, each starting exactly where the chapter starts.