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.

How to merge MP3 files into one audiobook

  1. Drop the complete set, in any order. Order is worked out for you: by disc and track tag when every single file has a track number, otherwise by natural filename sort. Sets that mix tagged and untagged files use the filename rule for all of them, and files with identical track numbers tie-break by name. Anything still wrong moves with the arrows on each row.
  2. Remove what shouldn't be in the book. Publisher zips often include a sample chapter, an interview or a duplicate opening-credits file. The remove button on a row drops that file from the build without touching it on disk, and the remaining rows renumber.
  3. Turn filenames into chapter names. When an MP3 has an ID3 title it is used verbatim. Without one, the filename is cleaned: extension off, leading number and separator off, underscores to spaces, first letter capitalized, so "07_three_bears.mp3" starts as "Three bears". Fix the ones that read badly; the rest can stay.
  4. Match the bitrate to the best file in the set. Everything is re-encoded to one AAC bitrate, which is what makes the merged file uniform. If the sources are 64 kbps, choose 64; if they are a mix, choose for the highest, since you can't buy back quality the low ones lack and you shouldn't throw away what the good ones have.
  5. Set the book fields and build. Title, author, narrator and year are pre-filled from the first file that has album or artist tags, and the first embedded picture becomes the cover. Correct anything the ripper got wrong, then build; the output plays as one item with a marker at every former file boundary.

Merging versus concatenating: where the chapter markers come from

A concatenation tool glues the audio and forgets where the joins were. Kataribe probes each file for its duration before encoding and adds those durations up, so chapter 12 starts at exactly the sum of the eleven files before it, to the millisecond. The markers are written into the MP4 chapter list, which is the structure players read for a chapter menu.

Nothing is trimmed or padded. If each story file ends with two seconds of silence, those two seconds belong to that chapter, and the next chapter starts when the next file's audio starts. There is no crossfade, no gap insertion and no silence detection; the file is the unit of structure.

What a merged book looks like to a player

One file, one library entry, one remembered position, and a chapter list that reads the titles you typed. Skipping forward by chapter jumps to the next story; the sleep timer in most apps can stop at a chapter end, which is exactly a story boundary here. Shuffle doesn't apply because the player sees a book, not a playlist.

The cost is that the structure is fixed at build time. Reordering stories later means rebuilding from the MP3s, so keep the originals, and put the effort into the order and titles now rather than in a player's editor.

Long sets: 40 stories or 400 lectures

There is no ceiling on file count in the builder. Files are staged through the engine one at a time and the chapter list grows with them, so a lecture series of several hundred short recordings builds the same way as a novel in a dozen parts. Total playing time and bitrate decide the memory the tab needs, not how many pieces it arrived in.

Very long chapter lists are fine in players but tedious to name by hand. If the source filenames are sensible, the cleaning rules produce usable titles; if they are opaque numbers, consider renaming the files in bulk on disk first, since the builder edits one title at a time.

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

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.
Can I merge MP3s without re-encoding them?
No. Merging without re-encoding is only possible when every input is already AAC; MP3 frames are always decoded and encoded to AAC. To minimize the loss, pick the bitrate the MP3s already have, or the next step up.
Do the MP3s all need the same bitrate or sample rate?
Bitrate, no: each file is encoded to the single bitrate you select, so a set that mixes 64 and 128 kbps sources comes out uniform. Channel layout is passed through per file, so keep mono and stereo from mixing inside one book.
What is the merged file called?
Author - Title.m4b, built from the two fields with characters that filesystems reject removed. Rename it however you like; players read the tags inside, not the name.
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.