What is a .m4b file?
MPEG-4 Audiobook · Audio · Apple / MPEG · 2003
An .m4b file is an MPEG-4 audiobook, the same container as M4A but with chapter markers and the ability to remember your place. It is the standard audiobook format in Apple's ecosystem.
What is a .m4b file used for?
M4B is an ISO base-media file (ftyp box at offset 4) holding AAC audio plus chapter and bookmark data. The extension tells apps to treat it as a resumable audiobook rather than a song.
Apple Books and many audiobook players read it. Some M4B files from stores carry DRM; DRM-free ones convert to MP3, though you lose chapters.
What opens a .m4b file?
The .m4b format opens in the following apps, grouped by operating system. If one app does not work, try another from the same list.
Windows
- VLC
- AIMP
- Audiobookshelf
macOS
- Apple Books
- VLC
- Audiobookshelf
Linux
- VLC
- Cozy
- ffmpeg
Open a .m4b file step by step
- Confirm the file really is a .m4b. Drop it into the WhatFileType identifier, which reads its signature in your browser without uploading anything.
- On Windows, open it with VLC or AIMP.
- On Mac, use Apple Books or VLC.
- On Linux, try VLC or Cozy.
- Still stuck? Convert the .m4b to mp3, which opens more widely.
Why can't I open my .m4b file?
Almost always because no installed app understands M4B. Install one of the apps above, or convert the file to a format your existing software already reads. If double-clicking opens the wrong program instead of nothing at all, the file association is set to the wrong app rather than the file being broken.
The other possibility is that the file is not really a .m4b at all. Drop it into the file identifier to check it by its bytes instead of its name. Nothing is uploaded.
How to convert a .m4b file
You can convert a .m4b file to mp3, m4a, aac using a conversion tool or the export menu of an app that opens it.
What is the .m4b file signature (magic bytes)?
A .m4b file carries the hex bytes 66 74 79 70 at offset 4, which read as ftyp in ASCII. Programs identify M4B by those bytes, not by the filename, so a renamed file still gives itself away.
These are the documented signatures for M4B. For a full breakdown, see the MP4 file signature page.
| Hex signature | Offset | ASCII | Note |
|---|---|---|---|
| 66 74 79 70 | 4 | ftyp | ftyp box (ISO base media) |
See the full magic-numbers reference for every format.
Frequently asked questions
What is a .m4b file?
An .m4b file is an MPEG-4 audiobook, the same container as M4A but with chapter markers and the ability to remember your place. It is the standard audiobook format in Apple's ecosystem.
How do I open an .m4b file?
Apple Books plays it on Apple devices; VLC and audiobook apps like Audiobookshelf open it elsewhere, keeping the chapters.
What is the difference between M4A and M4B?
Both are MPEG-4 audio. M4B adds chapters and remembers your position, and apps treat it as an audiobook rather than music.
Why won't my .m4b file play?
It may carry DRM from an audiobook store, which locks it to that store's app. DRM-free M4B files play in VLC and most audiobook players.
Related formats
Sources
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