Compress M4B
Shrink an existing M4B audiobook, or build a compact one from separate files — right in your browser. Pick a lower bitrate and download a much smaller file, with chapters and cover art preserved. Your files never leave your device.
Drop a finished M4B, or click to browse
M4B · M4A · one file · chapters and cover are preserved
How it works
- Step 1
Drop a finished M4B
Add one existing .m4b or .m4a file. It stays on your device — nothing is uploaded.
- Step 2
Pick a lower bitrate
See the current bitrate and the estimated new size, then choose how much to shrink.
- Step 3
Download the smaller file
Same chapters, cover art, and metadata — just a smaller file, ready for any M4B player.
Why M4B files get large — and how small they can be
File size comes down to three things: bitrate, whether the audio is mono or stereo, and the length of the book. Narration is mono speech, so a low bitrate sounds identical while taking a fraction of the space. Here is roughly what to expect at common audiobook bitrates.
| Bitrate | Channel | Per hour | 10-hour book | Best for |
|---|---|---|---|---|
| 64 kbps | Mono | ~28 MB | ~280 MB | Audiobook standard |
| 96 kbps | Mono | ~42 MB | ~420 MB | Speech + light music |
| 128 kbps | Stereo | ~56 MB | ~560 MB | Rarely needed for speech |
For plain narration, 64 kbps mono is the sweet spot: roughly half the size of 128 kbps stereo with no audible difference.
Compress M4B: common questions
- How small can an M4B audiobook get?
- It depends on bitrate and length. At 64 kbps mono — the standard for spoken word — expect roughly 28 MB per hour, so a 10-hour audiobook lands around 280 MB. That is typically 2 to 4 times smaller than the same book encoded at 128 kbps stereo, with no audible difference for narration.
- Does compressing an M4B reduce audio quality?
- For spoken-word audio, dropping to 64 kbps mono is essentially inaudible — human speech does not need stereo or high bitrates. For recordings with music or sound effects you may prefer 96 kbps. Compression re-encodes the audio, so going very low (below 48 kbps) can start to sound thin.
- What bitrate should I use for an audiobook?
- 64 kbps mono is the audiobook standard and the default here. Use 96 kbps if the recording has music or effects you want to preserve, and 128 kbps only if you have a specific reason — it roughly doubles the file size for no real benefit on narration.
- I already have a single .m4b file — can I shrink it directly?
- Yes. Choose the "Shrink an existing M4B" tab, drop your finished .m4b (or .m4a) in, and pick a lower bitrate. The tool re-encodes only the audio to a smaller AAC bitrate and keeps your chapter markers, cover art, and metadata intact.
- Are my files uploaded when I compress them?
- No. Everything runs inside your browser using a WebAssembly build of FFmpeg. Your audio never leaves your device. Open your browser's Network tab while converting to verify there are no upload requests.