Compress M4B
Reduce the file size of an 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 two things: the bitrate and the length of the book. Narration doesn't need a high bitrate, so a low one sounds the same while taking a fraction of the space. Here is roughly what to expect at common audiobook bitrates.
| Bitrate | Per hour | 10-hour book | Best for |
|---|---|---|---|
| 64 kbps | ~29 MB | ~290 MB | Audiobook standard |
| 96 kbps | ~43 MB | ~430 MB | Speech + light music |
| 128 kbps | ~58 MB | ~580 MB | Rarely needed for speech |
For plain narration, 64 kbps is the sweet spot: half the size of 128 kbps with no audible difference.
Compress M4B: common questions
- How do I compress an audiobook?
- Drop the M4B on this page, pick a bitrate (64 kbps suits narration), and click "Compress M4B". The smaller file downloads with its chapters, cover, and tags. Is the book a set of MP3s instead? Choose "Build from separate files" to turn them into one compact M4B.
- How small can an M4B audiobook get?
- It depends on bitrate and length. At 64 kbps, the usual bitrate for spoken word, an hour takes about 29 MB, so a 10-hour audiobook comes to about 290 MB. Many M4Bs are made at 128 kbps or more, so compressing them to 64 kbps halves the file or better, with no audible difference for narration.
- Does compressing an M4B reduce audio quality?
- For narration, 64 kbps AAC sounds the same as higher bitrates to most listeners; speech simply doesn't need more. For recordings with music or sound effects you may prefer 96 kbps. Compressing re-encodes the audio, so it can't restore quality the file never had.
- What bitrate should I use for an audiobook?
- 64 kbps is the usual audiobook bitrate 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 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.