M4B (MPEG-4 Audiobook)
Apple's audiobook format that remembers exactly where you stopped listening, down to the second.
| Full name | MPEG-4 Audiobook |
| Extension | .m4b |
| MIME type | audio/mp4 |
| Developer | Apple |
| Released | 2003 (alongside iTunes 4 and the iTunes Music Store) |
| Type | Lossy audio (AAC) in MPEG-4 container |
| Audio codec | AAC (Advanced Audio Coding), typically 64–128 kbps |
| Container | MPEG-4 Part 14 (ISO/IEC 14496-14) |
What is a M4B file?
M4B is Apple's file format for audiobooks and spoken-word recordings. It stores AAC-encoded audio inside an MPEG-4 container, adding chapter markers and a bookmark that tracks your listening position. When you close an M4B in Apple Books or iTunes, it saves your exact spot so you can pick up where you left off.
An M4B file is an audio-only MPEG-4 file with specific metadata that marks it as an audiobook. The audio is encoded with AAC, the same codec used by M4A music files. What sets M4B apart is its support for chapter markers, which lets players jump to named sections, and per-file bookmarking, which saves the playback position even after the app closes. M4B files can also carry cover art, author information, and embedded hyperlinks inside the metadata.
History
Apple introduced the M4B extension in 2003 alongside iTunes 4 and the launch of the iTunes Music Store. The format was built on MPEG-4 Part 14, an international standard (ISO/IEC 14496-14:2003) ratified that same year. Apple added the .m4b extension specifically to distinguish audiobook files from ordinary .m4a music files, allowing iTunes and iPod hardware to treat them differently, applying bookmarking and chapter navigation that made no sense for music tracks.
How it works
M4B files use the MPEG-4 Part 14 container, the same box-based structure that MP4 video files use. The audio data is stored in compressed AAC frames inside a media data box (mdat), while a separate movie box (moov) holds timing, chapter track data, and all metadata. Chapter markers are stored as a separate text track inside the container, giving compatible players named segments they can display in a chapter list. Cover art is embedded as a standard iTunes metadata atom, visible in any player that reads MP4 tags.
What it is used for
- Distributing audiobooks through Apple Books with chapter navigation and bookmarking
- Creating personal audiobooks from text-to-speech output or recorded narration
- Packaging long-form podcast episodes or lecture series with chapter markers
- Converting physical audiobook CDs into a single digital file with named chapters
How to open it
Apple Books and iTunes open M4B files natively on macOS and iOS, preserving bookmarks and chapter navigation. On other platforms, VLC opens M4B files and supports chapter navigation; fre:ac, Foobar2000, and Clementine also handle the format without extra plugins.
Pros and cons
Strengths
- Built-in bookmarking saves your position automatically, even if the app is closed
- Chapter markers let you jump to named sections without scrubbing through the file
- AAC audio at 64 kbps gives clear speech quality at a small file size
- Supports embedded cover art, author, narrator, and publisher metadata
Trade-offs
- Native playback support is strongest on Apple devices; Android and Windows need a third-party player
- AAC is lossy, so converting to a lossless format will not recover the removed audio data
- Older M4B files purchased from iTunes may carry FairPlay DRM, restricting playback to authorized Apple devices
- Not accepted by Audible or most non-Apple audiobook platforms, which use their own formats
Convert M4B files
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M4B FAQ
What is the difference between M4B and M4A?
Both M4B and M4A are MPEG-4 audio files using AAC encoding. The difference is metadata and playback behavior. M4B is flagged as an audiobook, so Apple devices apply bookmarking and chapter navigation. M4A is treated as a music track, with no bookmark saved between sessions. The audio quality at the same bitrate is identical.
Can I play M4B files on Android?
Android's built-in media player does not support M4B chapter navigation or bookmarking. Third-party apps do: Smart AudioBook Player, Voice Audiobook Player, and VLC all open M4B files on Android and support chapters.
Do all M4B files have DRM?
No. Only M4B files purchased from the iTunes Store before 2009 were protected with Apple's FairPlay DRM. Self-created M4B files and most modern purchases are DRM-free. If you created the file yourself or converted it from an unprotected source, there is no DRM.
How do I convert M4B to MP3?
You can convert M4B to MP3 using this converter or tools like VLC, fre:ac, or FFmpeg. Keep in mind that converting between two lossy formats (AAC and MP3) processes audio that has already been compressed, which can introduce minor quality artifacts. When possible, encode from the original source rather than converting from M4B.