Add chapters to an M4B audiobook

Chapters in an M4B are markers inside the file, and Kataribe writes them: one per input file. Drop the parts of a book — MP3, M4A, WAV, FLAC, OGG, or M4A/M4B parts — and get one .m4b whose chapter list matches the files, built in your browser.

A language course is a good example: 30 lesson files, each an M4A named lesson01 to lesson30. Dropped here, they become one book with 30 chapters, ordered naturally (lesson10 after lesson09), titled from the tags or filenames, and renamable to "Lesson 3: At the market" before building. Because the audio is already AAC, it's copied rather than re-encoded, so the build is quick and nothing is lost. Jumping between lessons in Apple Books or Audiobookshelf then works from the chapter list instead of scrubbing.

Two limits worth knowing. Chapters aren't detected from silence, so a single long file becomes a single chapter — to get real chapters, start from the per-chapter files. And an existing M4B isn't edited in place: dropping one rebuilds it with the title, cover and metadata you set, which is the way to fix a book with the wrong author or no cover, but the chapter structure still follows the files you drop.

How to add chapters to an M4B file

  1. Start from the pieces, not the finished file. Chapters are file boundaries, so collect the per-chapter sources: the MP3s a book was ripped from, the M4A parts a course shipped as, or WAV exports from a session. Any format the builder accepts can be mixed in one set.
  2. Name the files in play order. Numbered names sort naturally: Part 1 through Part 12 land in order without zero padding. Where numbers are absent (Prologue, Epilogue, Bonus), drop the files anyway and use the row arrows to slot them; position in the list is the only thing that decides chapter order.
  3. Write the chapter titles. For MP3 parts the ID3 title is the default; for M4A, M4B, WAV, FLAC or OGG parts the cleaned filename is, since those formats' tags aren't read. Whatever is in the text box when you build is what the player's chapter menu shows, so this is the moment to write "Unit 3: Ordering food" rather than fix it later.
  4. Re-enter the book details. Metadata inside the parts is stripped on purpose, and MP4 tags aren't read to begin with, so title, author, narrator and year need typing unless the parts are tagged MP3s. Add the cover as JPEG or PNG; an existing M4B's embedded art isn't carried over.
  5. Build, and check the mode in the result line. All-AAC inputs are copied and the result says so; a mixed set is encoded at the selected bitrate. The chapter count shown after the build should equal the number of files you dropped; if it doesn't, a file was filtered out because its extension wasn't one the builder accepts.

Chapter markers from filenames vs. embedded titles

Two sources feed the default chapter names. An ID3v2 title frame, present in most MP3s, wins outright and is used as-is. Otherwise the filename is processed: extension removed, a leading track, part, cd or disc number and its separator removed, underscores turned to spaces, and any run of words shared by every file in the set cut off up to the last separator.

That last rule is what turns "The Example Book - Part 1" through "Part 3" into "Part 1", "Part 2", "Part 3", and if a name is reduced to nothing, "Chapter N" stands in. The first letter is capitalized; nothing else is invented. Edited titles survive re-sorting when you add more files.

What happens to chapters and tags already in the parts

Each part goes through FFmpeg with its metadata and chapter atoms explicitly dropped before it is joined. That is deliberate: it stops a stale album title or an old chapter list from one part contaminating the book. It also means an M4B's internal chapters can't be preserved or merged; the file is treated as one block of audio.

The practical consequence is that fixing a chapterless M4B is a two-tool job when the sources are gone: split it into pieces elsewhere (at silences, at timestamps, wherever you decide), then bring the pieces here. Kataribe does the container work; it doesn't guess where chapters should fall.

How the chapter timestamps are computed

Before encoding, every file is probed for its duration. The chapter list is the running total: chapter one starts at zero and ends at file one's length, chapter two starts there, and so on, written with a millisecond timebase. The last chapter's end is the book's total length shown in the result.

Because the boundaries are computed from the files rather than detected in the audio, they are exact and reproducible: build the same files twice and the markers are identical. The only way to move a boundary is to move the audio, by trimming or splitting a source file before dropping it.

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

I have one long M4B with no chapters. Can Kataribe split it up?
Not on its own. Chapters are one per input file and there's no silence detection. If you still have the files the M4B was built from, drop those instead. Splitting an M4B into pieces is a different tool.
My M4B has the wrong title and no cover. Can I fix that?
Yes. Drop the M4B, change the title, author, narrator and year, drop in a cover, and build; the AAC audio is copied, not re-encoded. Chapters follow the files you drop, so if the original had chapters you want kept, rebuild from the per-chapter source files.
Can I rename the chapters?
Yes. Every chapter title is editable before you build. Defaults come from the ID3 title tag or a cleaned-up filename, so lesson07.m4a starts as "lesson07" and can be whatever you type.
Does re-chaptering M4A parts re-encode the audio?
No. When every input is AAC (M4A or M4B), the audio is copied into the new container and only chapter markers, cover and metadata are written. Mixed inputs are encoded at the bitrate you pick.
Can I add a chapter at a specific timestamp?
No. Chapter boundaries are the file boundaries and there is no timestamp editor. If you need a chapter at 12:34 inside one file, cut the file there with an audio editor first, drop both halves, and the marker falls at the cut.
Will the chapters show on every player?
The markers are standard MP4 chapter markers, which is what audiobook apps read: Apple Books, Audiobookshelf, Smart AudioBook Player, Voice, BookPlayer and VLC all display them. A plain music player that opens the file will play it but usually hides the list.
Can I merge two M4Bs that each have chapters into one?
You can join them, but each becomes one chapter; the inner chapter lists are dropped. For a two-volume omnibus with proper chapters, rebuild from both volumes' per-chapter files in a single drop.
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.