MP3 to M4B online
M4B is what turns a long recording into a book in the eyes of your player: audiobook apps shelve it separately and remember where you stopped.
Select the files you want to compress or convert and upload them here.
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Supported compression and conversion formats:
JPEG, JPG, PNG, GIF, WEBP, MP3, TIFF, TIF, BMP, DIB, RLE, JP2, JPF, JPG2, JPX, J2K, JPC, MJ2, OGG, OGV, SPX, WAV, 3GP, AAC, M4A, M4B, OPUS, OGA, MP4, AVI, MKV, MKA, SVG, EXR, RAW, ARW, SRF, SR2, CRW, CR2, CR3, DNG, DCR, KDC, ERF, 3FR, MEF, MRW, NEF, NRW, ORF, PEF, RAF, RWL, RW2, SRW, X3F, PDF, JXL.
Why the extension matters
M4B and M4A hold the same AAC audio — the difference is what software does with them. Apple Books, and most audiobook players on Android, treat .m4b as a book: it lands on the bookshelf instead of the music library, playback position is remembered between sessions, and speed controls apply.
That is why a long recording — a lecture series, a recorded reading, a podcast archive — is worth converting rather than leaving as MP3. As a music track, a six-hour file restarts from the beginning every time your player forgets about it.
What to expect from the conversion
The audio is re-encoded from MP3 into AAC, so this is a lossy-to-lossy step: pick the highest quality you can if the recording will be listened to closely. For narration at normal listening volumes, the change is not audible.
The conversion works on a file at a time and does not build chapter markers on its own — chapters come from the file you feed in. If you have a book split into dozens of MP3s, join them before converting rather than after.
Common questions
What is the difference between M4A and M4B?
The audio inside is the same AAC. The extension tells players to treat the file as an audiobook — separate shelf, remembered position — rather than as a song.
Will my player remember where I stopped?
If it supports audiobooks, yes: that behaviour is tied to the .m4b extension rather than to anything inside the file.
Can I convert several MP3s into one book?
Each file is converted separately here. To end up with a single book, join the parts into one MP3 first, then convert.