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.
How to convert MP3 files to one M4B audiobook
- Drop every MP3 of the book at once. Kataribe sorts by disc and track number only when every file carries a track tag; if a single file lacks one, the whole set falls back to natural filename order. Gaps in the numbering (01, 02, 05) don't matter, since the sequence is what counts. Use the up and down arrows on any row to fix stragglers.
- Check the chapter titles. Each chapter starts as the file's ID3 title; when there is none, the filename is used with its extension, leading number and any prefix shared by every file stripped, so "Dune - 03.mp3" becomes "03". Retype whatever you want to see in the chapter list.
- Pick a bitrate no lower than the source. Most MP3 audiobooks are 64 kbps mono or 96 to 128 kbps stereo. Choosing the same figure keeps the loss from this lossy-to-lossy step small; the encoder keeps the source channel layout, so a stereo file at 64 kbps has half that per channel.
- Confirm the cover and book details. The first file with embedded art supplies the cover; replace it with any JPEG or PNG, which is stored in the book unchanged. Title, author, narrator and year are read from the album, artist, composer and year tags of the first tagged file, and all four are editable.
- Build and keep the tab open. Each file is encoded in turn and the progress line names the file it is on. The download is called "Author - Title.m4b"; cancelling at any point terminates the engine and nothing partial is saved.
What changes between MP3 and M4B
MP3 is a bare stream of audio frames with an ID3 tag stapled to the front. M4B is an MP4 container: the audio is one track, the chapter list is a separate structure with millisecond timestamps, and the cover and book fields live in a metadata atom the player can read without scanning the audio.
The container is also why position memory works. Players store a position per file, so one M4B is one bookmark, while a folder of 90 MP3s is 90 bookmarks and a guess about which one you were in. Kataribe additionally sets the media-type atom to Audiobook, the flag apps use to keep the book out of music views.
The codec changes too, from MPEG-1 Layer III to AAC-LC, which is why this conversion always encodes while the M4A case can copy. Kataribe doesn't try to store MP3 frames inside the MP4 container; the .m4b convention is AAC, and that is what every audiobook app is tested against.
What happens to your ID3 tags
Per-file tags are dropped on purpose. Each MP3 is encoded to an AAC part with its metadata and any embedded chapter frames stripped, so stray comments, lyrics frames or a ripper's per-track album art don't end up in the book.
Book-level fields are rebuilt from the first file that has an album or artist tag: album becomes the title, album artist (or artist) the author, composer the narrator, and the year frame the year. The genre is written as Audiobook regardless of what the MP3s said, and there is no series or comment field, so a series name belongs in the title or in your player's own editor.
ID3 CHAP frames, if any of your MP3s carry them, are ignored. Chapters in the finished book come only from file boundaries, so a single MP3 that already has internal chapter frames becomes one chapter.
After you download the .m4b
Apple Books
- Mac: drag the .m4b into Books, or File → Add to Library. It lands under Audiobooks with the cover and chapters.
- iPhone / iPad: connect the device and sync audiobooks in Finder (or iTunes on Windows), or AirDrop the file and open it in Books.
- 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
- Copy the .m4b into your library folder as Author/Title/Book.m4b (or upload it from the web UI).
- Run a library scan; Audiobookshelf reads the embedded title, author, narrator, cover, and chapter markers.
- Chapters appear in the player on web, iOS, and Android — no chapter files or metadata edits needed.
Android players
- Copy the .m4b to the phone (USB, Drive, or Syncthing) into a folder your player watches.
- Smart AudioBook Player, Listen, Voice, and the Audiobookshelf app all read M4B chapters and cover art.
- One file per book means one entry in the library, with your position remembered across chapters.
Read the guide
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.
- How big will a 10-hour M4B be?
- At 64 kbps, close to 290 MB; the arithmetic is bitrate in kilobits divided by eight, times seconds. The same book is about 145 MB at 32 kbps, 430 MB at 96 and 575 MB at 128. Cover art adds its own file size on top, since the image is embedded as-is.
- Why does my audiobook show up as music?
- Because the files were tagged as music. An MP3 has no audiobook flag, and an M4A carries whatever the ripper wrote, which is usually the music type. Kataribe writes the MP4 media-type atom as Audiobook and uses the .m4b extension, which is what Apple Books and most players key on.
- Can I add more MP3s after the first batch?
- Yes. Drop them onto the same list and the whole set is re-sorted by the same rule; chapter titles you have already edited are kept, and book details you typed are not overwritten by the new files' tags.
- 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.