How big will it be? Size by length and bitrate
No file to hand? Type a book’s length to see its size at every common bitrate, and how many books like it fit in the space you have.
- 32 kbps monoSmallest; fine for speech on a phone speaker146 MB27 books like it in 4 GB
- 48 kbps monoSmall; everyday listening219 MB18 books like it in 4 GB
- 64 kbps mono for speechClear speech: the usual audiobook choice292 MB13 books like it in 4 GB
- 96 kbps monoExtra clarity438 MB9 books like it in 4 GB
- 128 kbps stereoMusic and sound design585 MB6 books like it in 4 GB
- 192 kbpsWhat ACX asks producers to upload (MP3)877 MB4 books like it in 4 GB
Audio at a constant bitrate, plus about 1.5% for the file’s structure; a cover image adds its own size. Sizes in decimal units, as storage is sold (1 GB = 1,000 MB). Drop a real file above to see its exact saving.
The best bitrate for audiobooks
For one voice, 64 kbps is the sweet spot, and the services that care about size agree:
- The ffmpeg project’s rule of thumb for AAC that sounds the same as the source is about 64 kbps per channel; narration is a single channel.1
- Kobo compresses the audiobooks authors upload to about 64 kbps, “so any higher fidelity is not necessary.”2
- LibriVox offers its recordings at 64 kbps and 128 kbps MP3.3
- ACX asks producers for 192 kbps MP3, but that is a master for the store to re-encode, not a listening copy.4
Go lower (32 or 48 kbps mono) to fit a long series on a full phone: speech stays easy to follow, with a little less air. Go higher only for books with music, sound effects or a full cast in stereo. AAC (M4B) sounds better than MP3 at the same bitrate, so prefer M4B unless a device needs MP3.
Why audiobooks compress so well
Music needs a wide range of frequencies and two channels to sound right. Narration is one voice, recorded in mono, with most of its information below 8 kHz. That is why speech stays clear at bitrates that would make music sound thin, and why an audiobook can often shrink to a third of its size with no difference you’d notice on headphones.
Size per hour
| Quality | Bitrate | Per hour | Good for |
|---|---|---|---|
| Smallest | 32 kbps mono | About 14 MB | Long books on a full phone, spoken word only |
| Small | 48 kbps mono | About 22 MB | Everyday listening |
| Voice | 64 kbps mono | About 29 MB | Close to the original for narration |
| Standard | 96 kbps mono | About 43 MB | Extra clarity |
| High | 128 kbps stereo | About 58 MB | Music and sound design |
Compressing an already compressed file always loses a little, so keep your original if you might want it again. To change the speed instead, try the speed changer; to make a quiet book louder, the volume booster.
Sources
- Encode/AAC, FFmpeg wiki
- How to upload your audiobook directly on Kobo, Kobo Writing Life Help
- Help and FAQ, LibriVox
- ACX audio submission requirements, ACX Help