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 put in natural filename order (FLAC tags aren't read yet, so keep disc and track numbers in the names), 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.

How to convert FLAC audiobook files to M4B

  1. Get the filenames into disc-then-track order. Track and disc numbers in FLAC live in Vorbis comments, which Kataribe does not read, so ordering is natural filename sort across everything you drop. Rippers that write "1-01 Title.flac" sort correctly as they are; per-disc folders that restart at "01 Title.flac" need a disc prefix added before discs 2 to 14 are dropped together, or a pass with the row arrows afterwards.
  2. Decide what the chapter titles should say. Defaults are the filenames minus extension and leading number, with any shared prefix removed, so "01 Chapter One.flac" becomes "Chapter One" and a bare "Track 07.flac" falls back to "Chapter 7". There is no bulk rename; either accept the defaults or retype the ones that matter.
  3. Pick the bitrate the FLAC deserves. This is a single-generation encode from lossless, so the result at a given bitrate is as good as AAC gets. 64 kbps for a mono or single-voice reading, 96 for stereo or a full-cast production, 128 only if music is a real part of the recording; anything above that spends space on nothing you can hear in speech.
  4. Cover from the rip folder. The FLAC picture block isn't extracted, so use the folder.jpg or cover.png the ripper saved, or scan the box yourself. JPEG and PNG are accepted and embedded unchanged.
  5. Build per volume for boxed sets. Each FLAC is decoded and encoded in turn, and the parts plus the finished book stay in the browser's memory until you download. For a set that runs several tens of hours, build one book per volume; the output size, not the FLAC size, is what eventually runs out of room.

Lossless sources: keep the FLAC, ship the M4B

The reason to hold a FLAC rip is that it can be re-encoded to any future format without stacking losses. Converting it to AAC now does not change that; the FLACs stay exactly as they were, and if 64 kbps turns out too tight for a particular reader's voice you rebuild at 96 from the same source, with no penalty from the first attempt.

The opposite direction is what to avoid: encoding the M4B and deleting the FLAC turns a lossless archive into a lossy one. Disk is cheap relative to re-ripping 14 discs. Treat the M4B as disposable and the FLAC folder as the master.

Why disc rips produce 300 chapters

CD audiobooks were cut into tracks of three to five minutes so that CD players could skip inside a disc, and a FLAC rip preserves every one of those cuts. Kataribe maps files to chapters one to one, so a 14-disc set becomes a book with as many chapters as tracks.

In practice that is workable. Players show a scrollable chapter list, skipping by chapter moves in small steps, and the position is remembered by time, not by chapter. If you want real chapters, the join has to happen before Kataribe, at the file level, because there is no chapter merging in the builder.

What the encoder sees from FLAC

FLAC is decoded to PCM first, so bit depth (16 or 24) and sample rate are handed to the AAC encoder as they come; mono stays mono and stereo stays stereo. Kataribe passes no resampling or downmix options, so a stereo rip of a mono broadcast produces a stereo AAC with two identical channels, which wastes half the bitrate.

Silence between tracks is kept as well. CD rips often carry a second or two of gap at track ends; those seconds land inside the chapter they belong to, so the timestamps stay accurate, but nothing trims them.

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 the FLAC tags used for the book title and author?
No. FLAC stores Vorbis comments and a picture block, and Kataribe only parses ID3v2 headers, which FLAC files normally don't have. Type the title, author and narrator, and drop the rip's folder image or a scan of the box as JPEG or PNG.
How many files can one build take?
There is no fixed cap; 300 tracks from a 14-disc rip is fine. What limits a build is memory in the browser tab, and that scales with the finished book's size rather than the file count, so a very large set is better built per volume.
Can I get one chapter per disc instead of per track?
Not from Kataribe alone, since every input file becomes exactly one chapter. Join each disc's tracks into one FLAC or WAV with another tool first, then drop 14 files here and you get 14 chapters.
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.