The short answer
For listening, M4B is usually the better audiobook format: one file per book, chapters that audiobook apps show, and AAC audio that sounds better than MP3 at the same size.1,3 MP3 wins on compatibility: “MP3 is supported by pretty much everything.”3 File size depends only on bitrate and length, so neither format is smaller by itself.
- One file per book
- M4B
- Plays everywhere
- MP3
- Same bitrate means
- Same size
- Better at low bitrates
- M4B (AAC)
Side by side
| M4B | MP3 | |
|---|---|---|
| Audio codec | AAC (usually) | MP3 |
| Quality at the same size | Higher | Lower |
| Files per book | One | Often one per chapter |
| Chapters | Yes | Possible; fewer players read them |
| Cover and tags | Yes | Yes |
| Plays on almost anything | Most phones and computers | Yes |
| Car stereos, old MP3 players | Only those that play AAC | Yes |
| Required by ACX | No | 192 kbps CBR |
File size: bitrate decides, not format
An audio file’s size is its bitrate times its length: a kilobit per second is 0.45 MB an hour, so 64 kbps is 28.8 MB an hour whether the file is MP3 or M4B. M4B files are usually smaller because AAC sounds good at lower bitrates, so audiobooks are encoded lower.1,2
Size of a 10-hour audiobook (MB = 1,000,000 bytes)
- Bitrate
- Your pick: 64 kbps = 288 MB
Sound quality
Audacity’s documentation says the M4A (AAC) and Opus options “deliver higher quality at the same file size compared to MP3”.1 Fraunhofer IIS, which co-developed MP3, says modern codecs such as AAC “can deliver more features and a higher audio quality at much lower bitrates compared to mp3.”2 For a single speaking voice, both formats sound clean at the bitrates audiobooks use; the difference shows when you squeeze the file small.
The ffmpeg project’s guide to the LAME MP3 encoder puts its default quality setting (V4, about 165 kbps for stereo) “close to perceptual transparency”.14 Speech in mono needs far less.
Chapters
M4B holds chapters in a chapter track that audiobook apps read.5 MP3 can carry chapters too, through ID3 “CHAP” and “CTOC” frames added to the standard in 2005,4 but support is patchier: BookPlayer reads them,9 while VLC 3 doesn’t.8 That is why MP3 audiobooks usually come as one file per chapter.
Compatibility
MP3 is the safe choice for devices: Audiobookshelf’s documentation says “MP3 is supported by pretty much everything”, while M4B “is supported on a number of devices, but does not have as good of support as mp3.”3 Windows lists AAC and M4A among its built-in formats, and Chromebook’s file list names .m4a but not .m4b.6,7 When a program won’t open an .m4b, renaming it .m4a often works, because it is the same format.
What about FLAC and Opus?
- FLAC is lossless.11 Use it for masters you’ll edit, not for listening copies: files are many times larger.
- Opus is “totally open, royalty-free”, works from 6 to 510 kbps and handles speech well.10 It’s efficient for your own library if your apps support it.
Which should you use?
- Listening on a phone or computer: M4B, in an audiobook app.
- A car stereo, an old MP3 player, a kids’ player: MP3, one file per chapter.
- Submitting to ACX: MP3 at 192 kbps or more, constant bit rate, one file per chapter.13
- Archiving a recording you’ll edit: WAV or FLAC.