What Is an M4B File? The Audiobook Format, Explained

What’s inside an M4B

The MP4 Registration Authority, which keeps the official list of MP4 file types, describes M4B as an “iTunes AudioBook protected or not, can contain audio + video + 3g text track + chapter track”.¹ In practice an audiobook M4B holds:

• The audio, almost always compressed with AAC, the codec Apple uses for music; it sounds better than MP3 at the same size.⁸
• Chapter markers, stored as a chapter track and often a second chapter list (ffmpeg writes both a QuickTime chapter track and “Nero” chapters by default).⁴
• A cover image and tags: title, author, narrator, series, year and description.

Wikipedia expands the extension as “MPEG-4 Audiobook”; the Library of Congress says .m4b is used for “bookmarkable” files.^2,3 Either way, the B means the file is meant to be a book: players treat it as one long book with chapters and resume where you stopped.

M4B, M4A and MP4: the same format

MP4, M4A and M4B are all the same MPEG-4 container. The only standard extension is .mp4; the others signal what’s inside.² The MP4 registry lists M4A with exactly the same description as M4B.¹ So you can rename an .m4b to .m4a for a player that doesn’t know the .m4b extension: the file doesn’t change. Renaming it to .mp3 does nothing useful: the audio is still AAC, and MP3-only players will refuse it.

AAX is Audible’s protected version: how to play it on each device, and why it can’t be converted, are in Audible AAX files.

Is M4B lossless? Is it good quality?

Most M4B audiobooks use AAC, which is lossy like MP3: some detail is discarded to make the file small. An MPEG-4 file can also hold Apple Lossless, which is “bit-for-bit identical to the original”, but that is rare for audiobooks because files get very large.⁵ For a single voice, AAC at 64 kbps is plenty: the ffmpeg project’s rule of thumb for transparent AAC is about 64 kbps per channel.¹⁵ Converting an MP3 audiobook to M4B can’t improve it: Apple notes that converting between compressed formats can reduce quality.⁶

Chapters and DRM

Most audiobook M4Bs have chapters, but not all. Apple’s own iPhone guide notes that “some audiobooks refer to chapters as tracks, or don’t define chapters.”⁷ If yours has none, you can add them without re-encoding; see audiobook chapters.

The format can be protected or not.¹ Audiobooks from Audible are protected and play only in Audible’s apps. Files you make yourself, LibriVox downloads, and audiobooks sold DRM-free (Libro.fm, for example) are ordinary M4B or MP3 files you can play anywhere. This site doesn’t remove DRM.

How to open and play an M4B

Because M4B is MPEG-4 audio, most devices can decode it: Android supports AAC in .mp4 and .m4a files, Windows lists AAC and M4A among its built-in formats, and every browser plays MP4 audio.^12,13,14 What the built-in players often lack is chapters and resume, so use an audiobook app:

• iPhone and iPad: BookPlayer (free, reads chapters) or Apple Books.
• Android: Voice (free, open source) or Smart AudioBook Player.
• Windows, Mac, Linux: VLC, or our browser-based player, which needs no install.

Step-by-step instructions for each device are in how to play M4B files.

Converting to and from M4B

• MP3 to M4B: join a folder of MP3s into one M4B with a chapter per file and a cover. See how to convert MP3 to M4B.
• M4B to MP3: for players and car stereos that only take MP3, as one file or one per chapter. See how to convert M4B to MP3.
• Which is better? See M4B vs MP3.