In depth
Everything you need to know about MP3 to M4B
Audiobooks often arrive as a folder of MP3s: one file per chapter, named 01.mp3 to 42.mp3. That plays, but most apps treat each file as a separate track, so the book has no chapter list of its own and a long listen is easy to lose track of. MP3 to M4B fixes that: the files become one .m4b with a chapter marker at the start of each.
The converter above does it in your browser. Drop the files, set the order and the chapter names, add a title and author, and download one M4B with chapters. There's nothing to install, no account, and your audio is never uploaded.
Merge MP3 to M4B in three steps
First, add the files. Drop them on the converter or choose them from your computer; it takes up to 200 files and 1.5 GB per book, which by our arithmetic is about 27 hours of audio at 128 kbps. The total running time and the estimated size of the M4B appear as soon as the files are read.
Second, check the list. Drag a file to a new position, use the arrow keys, or sort by file name, and type the title each chapter should show. Third, add the book title and author, choose voice quality (mono, 64 kbps) or music quality (stereo, 128 kbps), and create the M4B. A long book takes a while, because every file is decoded and encoded on your own computer, so keep the tab open until the download starts.
Each MP3 becomes a chapter: order, names and timestamps
The rule is one file, one chapter. A marker is written at the start of each file, so the timestamps come from the lengths of your MP3s, not from guesswork. If file 1 runs 12:40 and file 2 runs 18:05, chapter 2 starts at 12:40 and chapter 3 at 30:45.
That's why order and names matter most. Files from different sources are often numbered inconsistently, so a sort by name can still leave "Epilogue" at the top; check the list before you convert. Name chapters the way a listener would look for them, like "Chapter 3: The Lighthouse", because that's what the player's chapter list shows.
This is also the simplest answer to how to add chapters to an audiobook: split the audio into one file per chapter, then merge. If a chapter is spread over two MP3s, it gets two markers; call them "Chapter 7, part 1" and "part 2". If one MP3 holds the whole book, the converter can't find chapter breaks inside it, so split that file first.
Title and author: what the player shows
Type the book title and the author before you convert. They're saved in the M4B's tags, so audiobook apps list the file under the right book and author instead of a file name.
The converter doesn't add cover art. If you want artwork on your phone, set it in your library app afterwards; stores take the cover as a separate image upload anyway.
Why M4B: one file, chapter markers, resume position
M4B uses the same MPEG-4 container as .m4a, with AAC audio inside. The extension tells players to treat it as a book: show the chapter list, skip forward or back a chapter, and in most audiobook apps keep your place when you switch to music or a podcast.
One file is also easier to live with. It's one thing to copy to a phone, back up or move between apps, and the chapter markers travel inside it. A folder of MP3s is still the better choice for a few car stereos and stores that ask for a file per chapter, so keep your originals.
Private by design: files never leave your browser
Everything happens in the tab. The browser reads your files, decodes them, encodes the audio as AAC and writes the M4B in memory, then hands you the download. No audio, file name or tag is sent to our server. A slower computer simply takes longer, and a very long book needs free memory, so close other heavy tabs before you convert one.
Converting does re-encode the audio. MP3 and AAC are both lossy formats, so expect a small change in quality; keep your MP3s as the master copies. The converter reads DRM-free files only: protected downloads such as Audible's can't be opened, and it doesn't remove copy protection.
Playing the M4B on iPhone, Android and Audiobookshelf
On iPhone, Apple Books and players such as BookPlayer open the file and show its chapters. On Android, use an audiobook app such as Smart AudioBook Player; some music apps play the audio but ignore chapters. On a computer, VLC plays it with the chapter list.
To convert MP3 to M4B for Audiobookshelf, run the files through the converter and add the .m4b to your library, in a folder of its own. The server reads the markers, so its apps show the chapters on each device.
Starting from text instead? Make the M4B directly
If the book doesn't exist as audio yet, you don't need MP3s at all. FullCastBook turns a manuscript (EPUB, DOCX, PDF with a text layer, TXT or Markdown) into narration, gives every speaking character a distinct voice next to the narrator, and exports an M4B with chapter markers straight away. See the M4B audiobook maker or the AI audiobook generator; new accounts get free credits for a short sample chapter.
