Convert MP3 to M4B

Drop a folder of MP3s and get back one .m4b audiobook: AAC audio in an MP4 container with real chapter markers, cover art and book metadata. Kataribe builds it in your browser with FFmpeg compiled to WebAssembly, so nothing is uploaded.

The MP3s can be anything: a serialized audio drama downloaded episode by episode, a course, a book bought from a DRM-free store as a zip of tracks. As loose files they play, but Apple Books treats them as music with no chapter list and no remembered position, and phone players show a folder rather than a book. Kataribe reads the ID3 tags to pre-fill title, author, narrator and cover, orders the files by track number (or by natural filename order when tags are missing, so 10 comes after 9 rather than after 1), and makes each file a chapter with an editable title.

Pick a bitrate — 64 kbps AAC is the sensible default for spoken word, higher if the production has music — and build. Files are processed one at a time, so a full-length book works in a desktop browser, and the finished .m4b plays in Apple Books, Audiobookshelf, BookPlayer, Prologue, Plappa, Smart AudioBook Player, VLC and anything else that reads M4B.

After you download the .m4b

Apple Books

  1. Mac: drag the .m4b into Books, or File → Add to Library. It lands under Audiobooks with the cover and chapters.
  2. iPhone / iPad: connect the device and sync audiobooks in Finder (or iTunes on Windows), or AirDrop the file and open it in Books.
  3. Books remembers your position and shows the chapter list from the file — that's the whole point of M4B over a folder of MP3s.

Audiobookshelf

  1. Copy the .m4b into your library folder as Author/Title/Book.m4b (or upload it from the web UI).
  2. Run a library scan; Audiobookshelf reads the embedded title, author, narrator, cover, and chapter markers.
  3. Chapters appear in the player on web, iOS, and Android — no chapter files or metadata edits needed.

Android players

  1. Copy the .m4b to the phone (USB, Drive, or Syncthing) into a folder your player watches.
  2. Smart AudioBook Player, Listen, Voice, and the Audiobookshelf app all read M4B chapters and cover art.
  3. One file per book means one entry in the library, with your position remembered across chapters.

Frequently asked questions

What is an M4B file?
M4B is an MP4 audio file with AAC audio, flagged as an audiobook rather than music. The container carries chapter markers, cover art and book metadata, and players remember your position in it. It's the format Apple Books and most audiobook apps expect.
How are the chapters decided?
One chapter per MP3, in track-number order from the ID3 tags or, failing that, natural filename order. Titles come from the ID3 title tag or a cleaned-up filename, and you can rename any chapter before building.
Which bitrate should I choose?
64 kbps AAC for narrated books; that's what many commercial audiobooks are sold at, and a 12-hour book comes out around 350 MB. Choose 96 or 128 for audio drama or music-heavy productions, 32 or 48 if space matters more. Mono or stereo is kept from the source.
Does converting MP3 to AAC lose quality?
It's a re-encode, so some. For speech at 64 kbps and above the loss is small; the thing to avoid is going below the bitrate of the source. If your MP3s are 64 kbps, pick 64, not 32.
Are my audio files uploaded anywhere?
No. Kataribe runs FFmpeg compiled to WebAssembly inside your browser, so the files never leave your device. The engine (about 30 MB) downloads once and is cached; after that there is no account, no queue and no watermark.
How long does a full book take?
Encoding runs many times faster than real time on a modern laptop, but a full-length book still takes minutes, not seconds. Keep the tab open until the download appears. If every input is already AAC, the audio is copied rather than re-encoded, which is much quicker.
Is this really free?
Yes. Every input format, every bitrate, cover art and metadata, no sign-up, and nothing added to the audio.
Does it work with DRM-protected audiobooks?
No. DRM-protected files (Audible .aax and similar) won't open, and we don't help with that. Kataribe is for audio you already have in plain MP3, M4A, WAV, FLAC or OGG.