Convert M4A to M4B

M4A and M4B are the same container with the same AAC audio inside; the difference is the extension, the audiobook flag and the chapters. Kataribe joins your M4A files into one .m4b, copying the audio untouched, and adds chapter markers, cover and metadata.

The usual route here: iTunes' "Create AAC Version" turned a folder of files into M4As, and Apple Books still lists them as separate tracks with an empty chapter menu. Or a book arrived as Part 1 through Part 8 in .m4a. Either way the audio is already AAC, so nothing needs re-encoding — Kataribe remuxes it, which is lossless and much faster than a re-encode. Each M4A becomes a chapter, ordered by track number or natural filename order, titled from the tag or the filename, and editable before you build.

Book title, author, narrator and cover are pre-filled from the tags where present, and all of it can be changed. If you mix in a non-AAC file such as an MP3 or WAV, the book is encoded at the bitrate you choose instead, so the output stays consistent.

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's the difference between M4A and M4B?
Nothing in the audio. Both are MP4 containers holding AAC. The .m4b extension tells Apple Books and audiobook players to treat the file as a book: show the chapter list, remember the position, and keep it out of music shuffle.
Is there any quality loss?
No, when every input is M4A or AAC. The audio stream is copied into the new container as it is; only chapters, cover and metadata are written. Re-encoding happens only if you add a file in another format.
Can't I just rename .m4a to .m4b?
For a single file, yes, and most players will accept it. Renaming doesn't join several files into one book or add chapter markers, which is what Kataribe does.
My M4As came from iTunes' Create AAC Version. Why are there no chapters?
That command converts one track into one M4A and writes music tags. It never joins files or creates chapter markers. Drop those M4As here and each track becomes a chapter in one book, without re-encoding.
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.