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.

Chapter titles come from the filenames and the book title, author, narrator and cover are typed in once (M4A tags aren't read yet), and all of it can be changed before you build. 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.

How to convert M4A files to a single M4B

  1. Name the files so they sort. MP4 files keep their tags in iTunes-style atoms, which Kataribe's tag reader (ID3v2 only) does not open, so the order comes from natural filename sorting: Part 2 lands before Part 10 without zero padding. If the names carry no numbers at all, reorder with the row arrows before building.
  2. Watch the progress line for "Copying". During the build each file is reported as Copying or Encoding. Copying means the AAC stream is being remuxed byte for byte; the bitrate you selected is not applied, and the finished file reports "AAC copied, no re-encode".
  3. Rename the chapters. With no readable tags, every chapter title starts as the cleaned filename with any prefix shared by all files removed, so "Dune - 04 Arrakis.m4a" starts as "04 Arrakis", while a file called "Part 4.m4a" is reduced to nothing and falls back to "Chapter 4". Type the names you want the player to show.
  4. Type the book details and choose a cover. Title, author, narrator and year start empty for the same reason; the M4A's embedded artwork is not extracted either. Add a JPEG or PNG cover from disk. It is written into the file without resizing, so a 4000-pixel scan adds its full weight to the book.
  5. Build. Copy mode is quick because nothing is decoded; a long book is mostly file I/O plus the final mux. Keep non-AAC files out of the set unless you accept a re-encode of everything.

Remux, not re-encode: what copy mode actually does

Each M4A is passed through FFmpeg with the audio stream copied into a fresh .m4a part, minus its old metadata and chapter atoms. The parts are then concatenated with the concat demuxer, again without decoding, and the chapter list, cover and book fields are muxed in. No sample of audio is touched at any point.

Because nothing is decoded, every file keeps its own bitrate, sample rate and channel count inside the finished book. That is fine when the parts came from one source; if you are joining an 80 kbps mono part to 256 kbps stereo ones, the join is still valid MP4 but the quality steps audibly at the boundary.

The decision is per build, not per file. One non-AAC input turns copy into encode for everything, which matters if you have one bonus track in MP3: build it separately or accept that the whole book is re-encoded at one bitrate.

Why your M4A tags don't pre-fill the form

Kataribe reads ID3v2 headers, the tag block at the front of an MP3. MP4 files store the same information as atoms inside the moov box (©nam, ©ART, aART, ©wrt, ©day, covr), and that parser isn't part of the builder today, so the fields stay blank and chapters default to filenames.

For an iTunes-converted set that is a few seconds of typing: one title, one author, one narrator. If the filenames are opaque, open the folder in a file manager that shows the Title column and rename the files first, since the filename is the only thing the chapter titles can be derived from.

The audiobook flag: what separates M4A from M4B in a player

Kataribe writes two things beyond the audio: the .m4b extension and the iTunes media-type atom set to Audiobook. Apple Books routes files on the extension and the flag; the Music app on macOS and iOS treats an .m4a without the flag as a song, which is where iTunes-converted books end up.

Android players mostly scan by extension and container, so a renamed .m4a often works there already, but the chapter list and the single-file position still depend on the merge this page performs. If a player lists your book under music after the build, check that the file kept its .m4b extension on the way over.

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.

Read the guide

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.
Does copy mode work for .m4b files too?
Yes. An M4B is the same container, and if it probes as AAC it is copied like any M4A. Its own chapter list and tags are stripped in the process, so what you get is the audio with the chapters and details you set here.
My M4A is Apple Lossless. Is it still copied?
No. Copy mode requires the audio stream to probe as AAC, and an ALAC file probes as alac, so the build switches to encoding at the bitrate you chose. Everything else in the same set is encoded too, since the whole book uses one mode.
Why is the bitrate menu still shown when nothing is encoded?
Because the decision is made at build time from the probe, after you have dropped the files. In copy mode the menu is ignored and the result line says "AAC copied, no re-encode"; if it shows a kbps figure instead, one of the inputs was not AAC.
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.