Turn DRM-free M4B audiobooks into MP3 files that play on any device. Title, author, narrator and cover art are carried over, chapters become ID3 chapter markers, or you can split the book into one numbered MP3 per chapter. Conversion uses LAME in your browser, with ffmpeg.wasm for long books and VBR.
Last updated:
Bitrate mode
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How to use the M4B to MP3 Converter
Add your M4B files. Cover, title, author, length and the chapter list appear right away.
Choose the quality: 128 kbps CBR is plenty for speech; use 192 kbps or VBR V2 for books with music.
Decide on chapters: tick Split into one MP3 per chapter, or keep a single MP3 with chapter markers.
Press Convert to MP3 and watch the progress of each book.
Download the MP3, the chapter ZIP, single chapters, or all books as one ZIP.
MP3 size per hour of audiobook
Setting
Average bitrate
Size per hour
10-hour book
Best for
128 kbps CBR, mono
128 kbps
≈ 58 MB
≈ 576 MB
Narration only, maximum compatibility
VBR V5
≈ 130 kbps
≈ 59 MB
≈ 585 MB
Speech with occasional music
VBR V2
≈ 190 kbps
≈ 86 MB
≈ 855 MB
Full-cast productions, music
192 kbps CBR
192 kbps
≈ 86 MB
≈ 864 MB
Music-heavy books, older car stereos
320 kbps CBR
320 kbps
≈ 144 MB
≈ 1.44 GB
Archiving, no compromise
MP3 size = bitrate × duration ÷ 8. For 128 kbps: 128,000 bits/s × 3,600 s ÷ 8 = 57.6 MB per hour. Converting from AAC to MP3 is lossy-to-lossy, so choose a bitrate at least as high as the source's (shown in the file details) to avoid audible loss.
How chapters are handled
M4B files store chapters in one of two ways: a QuickTime chapter track (a small text track with one sample per chapter title, used by iTunes and most modern tools) or a Nero chapter list (a chpl box with start times in 100-nanosecond units). This tool reads both and uses the more complete list. Each chapter ends where the next one starts, and the last chapter ends at the end of the book.
Single MP3: the chapters become ID3v2 CHAP frames with start and end times in milliseconds plus a CTOC table of contents (up to 255 chapters), which many podcast and audiobook apps display.
One MP3 per chapter: files are named 01 - Chapter title.mp3, tagged with the chapter title as title, the book title as album and "n/total" as track number, so they sort and play in order everywhere.
Which engine is used?
Automatic picks the browser engine for CBR when the decoded book fits comfortably in memory (roughly 75 minutes of 44.1 kHz stereo, or several hours of 22 kHz mono). Longer books and all VBR conversions use ffmpeg.wasm, a WebAssembly build of ffmpeg with the LAME encoder, which streams through the file instead of decoding it all at once. It is downloaded once (about 31 MB) and cached by your browser.
Frequently asked questions
What is an M4B file?
M4B is the audiobook version of the MP4/M4A format. It usually contains AAC audio plus metadata such as the title, author and cover, a list of chapters, and a flag that lets players remember your position. Many players outside the Apple ecosystem handle MP3 better, which is why people convert M4B to MP3.
Can I split an M4B audiobook into chapters?
Yes. Tick Split into one MP3 per chapter. The tool reads the chapter list embedded in the file and saves each chapter as its own MP3, named with a track number and the chapter title, with the book title as album and the chapter number as track number. If a book has no chapter list, it is converted as one file.
Which bitrate should I use for audiobooks?
Spoken word sounds clear at 64 to 96 kbps, so 128 kbps CBR (the lowest option here) is already generous, and Mono halves the size again. Choose 192 kbps or VBR V2 if the book contains music or sound effects. At 128 kbps an MP3 takes about 58 MB per hour.
What is the difference between CBR and VBR?
CBR (constant bitrate) uses the same number of bits every second, so file size is predictable and every player can seek accurately. VBR (variable bitrate) spends more bits on complex passages and fewer on silence, giving better quality per megabyte. LAME's V0 is about 245 kbps and V5 about 130 kbps on average.
Will the tags and cover art be kept?
Yes, when Keep tags and cover art is on. Title, author, album artist, narrator, year, genre, copyright, description and the embedded cover are written as ID3v2.3 tags. In single-file mode the chapters are also stored as ID3 chapter frames (CHAP and CTOC).
Can it convert audiobooks with DRM?
No. Files bought from stores that protect them with DRM (encrypted audio, such as Audible AAX or protected iTunes audiobooks) cannot be decoded, and the tool tells you when a file is encrypted. DRM-free M4B files, for example from libraries of public-domain recordings or your own ripped CDs, work.
Are my audiobooks uploaded?
No. The files are read, decoded and encoded on your own device. Only the converter engines (LAME.js, and ffmpeg.wasm when needed) are downloaded from a public CDN.
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.