Convert FLAC to M4B

FLAC is the right format for keeping an audiobook rip and the wrong one for listening on a phone. Kataribe turns a folder of FLAC files into one .m4b — AAC audio, chapter markers, cover, metadata — in your browser, and leaves the FLACs as your archive.

Say you ripped a boxed set losslessly years ago for safekeeping: 14 discs, 300 tracks, 4 GB, and it plays nowhere convenient. Apple Books doesn't open FLAC at all, and the apps that do show 300 tracks with no book structure. Drop all the files here at once; they're ordered by track number or, failing that, natural filename order, and each becomes a chapter. Disc-track titles can be left as they are or renamed before you build.

Choose a bitrate — 64 kbps for narration, higher if you want the headroom since the source is lossless — and build. Encoding happens one file at a time, so a large set fits in a desktop browser; a collection over about 10 GB is better built in parts. The output plays in Apple Books, Audiobookshelf, BookPlayer, Prologue, Smart AudioBook Player and VLC.

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

Isn't converting FLAC to AAC a step down?
Yes, by design. FLAC is the archive; the M4B is the listening copy. At 64 kbps AAC, narration is compact and plays anywhere, and the FLACs stay on disk untouched.
Can I keep the FLAC audio inside the M4B?
No. M4B means AAC in an MP4 container; that's what Apple Books and audiobook players expect. Keep the FLACs as the archive and treat the M4B as the copy you carry.
How is the order decided for FLAC files?
By track number when Kataribe can read one, otherwise by natural filename order — 01, 02, 10, not 1, 10, 2. The chapter list shows the order before you build; if the filenames don't sort the way you want, rename them and drop the files again.
How big will the M4B be?
Roughly hours times bitrate. At 64 kbps, an hour is about 29 MB, so a 20-hour boxed set comes out around 575 MB against several gigabytes of FLAC. Doubling the bitrate doubles the size.
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.