Compress audiobooks online
An audiobook is speech, and speech does not need music bitrates. That is why M4B files shrink further than almost anything else you own.
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 audiobooks compress so well
M4B is the audiobook flavour of AAC. Publishers encode at 64–128 kbps in stereo, which is generous for a narrator reading in a quiet studio: a human voice occupies a narrow frequency band and has no stereo image to preserve.
Dropping to 32–64 kbps mono keeps the narration completely intelligible while a ten-hour book falls from around 600 MB to 150–200 MB. On phone speakers and earbuds — where audiobooks are actually consumed — the difference is hard to notice; on studio monitors you would hear it.
When it is worth doing
Filling a phone with a library rather than one book. Fitting a long book onto an old player or a small card. Moving a collection to a tablet used on flights. Sending a recording to someone whose mailbox has a size limit.
One caution worth stating plainly: re-encoding rebuilds the audio stream, and chapter markers or bookmarks are not guaranteed to survive it. If your player relies on chapter navigation, keep the original file and treat the compressed copy as a travel version.
| Bitrate | Per hour | 10-hour book | Result |
|---|---|---|---|
| 128 kbps stereo | 55 MB | 550 MB | typical publisher encode |
| 64 kbps mono | 27 MB | 270 MB | indistinguishable on earbuds |
| 48 kbps mono | 21 MB | 210 MB | clear narration, small file |
| 32 kbps mono | 14 MB | 140 MB | audible thinning, still intelligible |
Common questions
How much smaller can an audiobook get?
Three to four times, typically. A 600 MB book at publisher settings lands between 150 and 200 MB at speech bitrates without becoming hard to follow.
Will the narration still be clear?
Yes, down to about 32 kbps mono. Speech survives low bitrates far better than music because it uses a narrow frequency range — that is exactly what makes audiobooks the best case for compression.
Do chapters survive compression?
Not reliably. Re-encoding rebuilds the stream and chapter markers may be lost, so keep the original if your player depends on them.
Can I compress several books at once?
Yes — upload them together or as a ZIP archive, and the result comes back as an archive.