Merge MP3 files into one audiobook

Combine any number of MP3 files into one audiobook file. Kataribe joins them in order, marks where each file began as a chapter, adds cover and book details, and writes a single .m4b — in your browser, with nothing uploaded.

Take a set of children's story recordings: 40 short MP3s, one per story, that a kid has to scroll through on a phone and that shuffle into nonsense the moment someone taps the wrong button. Merged into one .m4b, it's one item in the library, each story is a named chapter to jump to, and the player remembers where bedtime stopped. Files are ordered by ID3 track number or natural filename order, and every chapter title is editable — "07_three_bears" can become "The Three Bears" before you build.

This isn't a plain concatenation into one long MP3, which would lose the boundaries between stories. The output is AAC in an MP4 container with real chapter markers, which Apple Books, Audiobookshelf, BookPlayer, Prologue, Smart AudioBook Player and VLC all read. The desktop tools that used to do this are mostly Windows-only or no longer maintained; this one needs a modern browser and nothing else.

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

How do I control the order the files are joined in?
Track numbers from the ID3 tags come first; without them, natural filename order applies, so 2 comes before 10 even without leading zeros. The chapter list shows the final order before you build; rename files and drop them again if it's wrong.
Why M4B rather than one long MP3?
Chapter markers in MP3 are rarely read by players, and one long MP3 is just a long track. M4B chapters are standard MP4 markers that every audiobook app reads, and the file is treated as a book with a remembered position.
Does the merged file keep the MP3s' cover and tags?
Yes, when present: album becomes the book title, album artist or artist the author, composer the narrator, and embedded cover art the cover. All of it is editable, and you can drop in a different cover image.
Can I merge several different books at once?
No. One book per build. Drop the files for one book, build and download it, then start the next. Very large single books (10 GB or more of input) may need to be built in parts.
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.