Format guide
M4B vs MP3: the short answer
M4B vs MP3 is less a question of sound than of where the file is going. An M4B holds a whole audiobook in one file with a chapter list, and audiobook apps keep your place in it. An MP3 plays on nearly everything, and it's the format stores ask for when you upload, usually one file per chapter.
So for most authors the practical answer to the M4B vs MP3 audiobook question is both: the M4B for listeners and direct buyers, MP3 chapter files for stores. Below: what each format is, the file size and quality trade-off, which players and stores take which, and how to get both from one project. To make the file itself, see the M4B audiobook maker.
What is M4B? An AAC audio file with chapters and bookmarks
What is M4B, technically? An MPEG-4 audio file, the same container as .m4a, normally carrying AAC audio. The Library of Congress's MP4 format description notes that the M4B extension was used for bookmarkable files. The extension is a signal: an audiobook player sees .m4b and treats the file as a book, not a song.
Inside, an M4B can carry chapter markers: a timestamp and a title for each chapter. Players read them to show a chapter list and to skip a chapter forward or back. Because the markers live in the file, they travel with it to every device, along with the title and author tags.
M4B vs MP3: chapters, resume position, file size, quality
Chapters: an M4B's chapter list is what audiobook apps expect. MP3 has a chapter standard too, the CHAP and CTOC frames of the ID3 chapter addendum, and FullCastBook's whole-book MP3 includes them, but many music players and car stereos skip them. That's why MP3 audiobooks are usually split into one file per chapter: the file list becomes the chapter list.
Resume position: dedicated audiobook apps track progress in either format. With a general music player, results vary from app to app. M4B was designed around bookmarking, so for one long file it's the safer choice.
File size and quality, the m4b vs mp3 file size question: AAC, the codec inside most M4B files, is the more efficient of the two. In a paper published by Fraunhofer IIS, Karlheinz Brandenburg reports that AAC reaches on average the same quality as MP3 at about 70% of the bit rate. For a single voice, size mostly follows the bit rate. FullCastBook's M4B is mono AAC at 64 kbps, about 29 MB per hour by our arithmetic; its MP3 is 192 kbps constant bitrate, about 86 MB per hour, because that's the spec stores ask for. A 10-hour book is roughly 290 MB as M4B and 860 MB as MP3.
Which players handle M4B
On iPhone, audiobook apps such as BookPlayer list M4B among their formats and show chapters. On Android, the open-source Voice player organizes M4B and MP3 audiobooks with chapter navigation. On Windows and Mac, VLC opens M4B and lists chapters under Playback > Chapter. Device-by-device steps, and what to do when chapters don't show, are in how to play M4B files.
MP3 needs no such list: it's the one audiobook file format almost every device plays. If a listener says your M4B won't open on an old car stereo or a basic player, send them the chapter MP3s.
Which stores and platforms want MP3 per chapter
Most upload forms want MP3 chapter files, not M4B. As of October 2026, Spotify for Authors takes MP3, WAV or FLAC. Kobo Writing Life accepts .mp3 only, up to 200 MB per file and 2 GB per book. ACX asks for one chapter per file as 192 kbps or higher constant-bitrate MP3, though it doesn't accept AI narration unless Audible authorizes it. Google Play Books lists MP3, M4A with AAC, FLAC and WAV, and BookFunnel's audiobook delivery takes chapter-length MP3s. None of them lists M4B.
That's why FullCastBook's MP3 export is 192 kbps constant bitrate at 44.1 kHz: it matches the MP3 bitrate Spotify for Authors and ACX specify. Where to send the finished files, and each store's AI-narration rule, is covered in where to sell AI audiobooks.
Other audiobook formats: M4A, AAX, WAV, FLAC
Among audiobook formats, M4A is the same container as M4B with a music-file extension, so some players file it under songs. AAX is the format Audible delivers purchased books in; it's not something you produce. WAV and FLAC are uncompressed and lossless: Spotify and Google Play accept them, Kobo doesn't, and for a finished spoken-word book they mostly add size.
Getting both from one project, and converting
You don't have to choose before you produce. In FullCastBook one project exports an M4B with chapter markers, one MP3 of the whole book, or a ZIP with an MP3 per chapter, numbered in reading order. The M4B for listeners and the MP3s for a store come from the same master. Check the running time first with the audiobook length calculator; the files come from the AI audiobook generator.
Converting M4B to MP3, or MP3s into an M4B, is possible with tools such as ffmpeg or m4b-tool, but each lossy-to-lossy conversion costs a little quality and the chapter markers have to be rebuilt. Exporting each format from the source avoids both.
