Turn a folder of MP3 chapter files into a single M4B audiobook that remembers your place and shows a chapter list in Apple Books, BookPlayer, Smart AudioBook Player and other audiobook apps. Chapters are named from the filenames, and you can add the cover, title, author and narrator. The audio is processed on your own computer.
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Chapters
Book details
No cover
Square JPG or PNG, ideally 1400×1400 or larger.
Audio
0%
Your audiobook is ready
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How to use the M4B Audiobook Maker
Add your chapter files: select all MP3s of the book at once. They are sorted by filename and each one becomes a chapter.
Check the chapters: rename titles, reorder with the arrows, or press Number titles for Chapter 1, Chapter 2…
Fill in the book details and choose a cover image. Album and artist tags from the first MP3 are filled in for you.
Pick the audio quality: 64 kbps mono is ideal for narration; use stereo for music or dramatized productions.
Press Create M4B audiobook, wait for the progress bar, listen to a bit in the player, then press Download M4B.
How chapter names are made from filenames
With Clean names from filenames switched on, the extension, leading track numbers, "Track" prefixes and underscores are removed and the first letter is capitalised. If a filename is only a number, it is kept as is so you can use Number titles instead.
Filename
Chapter title
01 - Opening Night.mp3
Opening Night
02_the_storm.mp3
The storm
Track 03. A Quiet Harbor.mp3
A Quiet Harbor
Chapter 4.mp3
Chapter 4
05.mp3 (then Number titles)
Chapter 5
Files are sorted naturally, so Chapter 2 comes before Chapter 10 even without leading zeros.
Audio quality and file size
AAC is more efficient than MP3, so an M4B at a lower bitrate sounds as good as the MP3 source. Size is simply bitrate × length: 1 kbps is 450 KB per hour.
Setting
Size per hour
Good for
32 kbps mono
≈ 14 MB
Long non-fiction, maximum storage savings
48 kbps mono
≈ 21 MB
Narration on phones and earbuds
64 kbps mono
≈ 29 MB
Recommended for most single-narrator books
96 kbps stereo
≈ 43 MB
Books with music or sound effects
128 kbps stereo
≈ 58 MB
Full-cast and dramatized productions
What's written into the file
The tool runs ffmpeg with a concat filter that joins all chapters into one continuous AAC stream, resampled to 44.1 kHz, and reads an ffmetadata file with a [CHAPTER] entry for every chapter (millisecond start and end times). The MP4 muxer stores the chapters both as a QuickTime chapter track and as a Nero-style chpl list, so both Apple and non-Apple players find them. The cover is embedded as attached artwork (covr), and the book details become the standard title, album, artist, album artist, composer (narrator), year, genre and description tags.
Frequently asked questions
What is an M4B file?
M4B is an MPEG-4 audio file, the same format as M4A, with AAC audio inside. The .m4b extension tells players to treat it as an audiobook: they remember your playback position, show the embedded chapter list and cover, and often allow speed control and sleep timers.
Why combine MP3s into one M4B?
Audiobooks ripped from CDs or downloaded as MP3 often come as dozens of separate files that players treat like a music album, losing your place or shuffling tracks. A single M4B keeps the whole book together with named chapters, cover art and bookmarks, and AAC at 64 kbps usually makes it smaller than the MP3s.
How are the chapters created?
Each audio file becomes one chapter. The chapter title comes from the filename with leading track numbers, underscores and the extension removed, so 03_The_Long_Road.mp3 becomes The Long Road. Start times are calculated from the exact length of each file, and you can rename or reorder chapters before creating the M4B.
What bitrate should I use for an audiobook?
For a single narrator, AAC at 64 kbps mono is clear and uses about 29 MB per hour; 48 or 32 kbps mono still sounds acceptable for speech and saves space. For music, sound effects or full-cast productions, use 96 to 128 kbps stereo. Choosing a higher bitrate than the source MP3s does not improve quality.
Will the M4B work on iPhone, Android and Windows?
Yes. Apple Books and the iOS Files app open M4B files, Android apps such as Smart AudioBook Player, Voice and BookPlayer-style players read the chapters and cover, and VLC, foobar2000 and iTunes play them on computers. Chapters are written in the standard MP4 chapter formats that these apps use.
Is there a limit to how big the audiobook can be?
The conversion runs inside your browser, which can give it roughly 2 GB of working memory. Books whose MP3 files add up to about 1.5 GB (more than 25 hours at 128 kbps) work on most computers. For very long books, create the M4B in two parts.
Are my audio files uploaded?
No. The tool uses ffmpeg compiled to WebAssembly, which runs inside the browser tab. Your files are read from your disk and the M4B is created on your own computer; only the converter program itself is downloaded once.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.