EPUB to M4B
M4B is the format audiobooks are actually meant to be in: one file, chapter marks, and a player that remembers where you got to. This makes one out of an EPUB without the book leaving your computer.
Local narration unit
Idle
Drop an EPUB here
M4B
With chapters
0
Bytes uploaded
11
Voices
Read aloud in this tab. Never uploaded. Here's why that's true.
book.epub
- —
- Chapters
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- Audio
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- On this machine
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- Output
Reading the book…
—
Close the tab if you want. We keep your place, and picking the same file again carries on from where it stopped.
Paused
Your place is kept.
Your audiobook is ready
Getting it onto a phone
- iPhone. AirDrop it, or copy it into the Books app. Chapters and your position work straight away.
- Android. Copy it across and open it in Smart AudioBook Player, Voice or Listen.
- Plex or Audiobookshelf. Drop it in the library folder.
That didn't work
- M4B
- One file, chaptered
- 64 kbps
- Mono, tuned for speech
- ~29 MB
- Per hour of audio
Why M4B rather than a folder of MP3s
An MP3 is a file of audio and nothing else. Put twenty of them in a folder and a player will usually play them in order, but it has no idea they are one book. Close the app, come back tomorrow, and plenty of them will have forgotten where you were.
M4B carries that information inside the file. Chapter marks let you skip a chapter back when you lose the thread, which is the thing people actually do while listening. And because players recognise the format as an audiobook rather than as music, they keep your position, remember your playback speed and keep the book out of your shuffle.
What we put in the file
- Chapters, taken from the book’s own table of contents rather than guessed.
- Cover art, lifted out of the EPUB and re-encoded so every player accepts it.
- Title and author, from the book’s metadata.
- The audiobook media flag, which is what tells Apple Books to file it under Audiobooks instead of dropping it into your music library.
Chapter markers get written twice, in two different conventions, because players disagree about which one to read. Apple Books and CarPlay want one; a lot of Android and desktop software wants the other. Writing both is what makes the same file work everywhere.
Getting it onto a phone
On an iPhone, AirDrop the file or copy it into the Books app. On Android, copy it across and open it in Smart AudioBook Player or Voice. For a media server, drop it in the library folder and let it index. Nothing needs converting again at the other end.
What it costs you
Time. The narration is generated on your own machine, so a ten-hour book takes roughly three an hour on an ordinary laptop, with no graphics card involved. We measure your machine before you commit and show you the real number. In exchange, the book is never uploaded and nobody else ends up with a copy of it.
Questions
What makes M4B different from an MP3?
Two things a listener notices. It holds chapter marks, so you can skip by chapter instead of scrubbing, and players treat it as an audiobook rather than as music, which means it remembers where you stopped. An MP3 folder can do neither on its own.
Which players open M4B?
Apple Books on iPhone, iPad and Mac. On Android, Smart AudioBook Player, Voice, Listen and BookPlayer. On the desktop, VLC, Foobar2000 and Plex. Audiobookshelf and Jellyfin both index them properly.
Will it show the cover?
Yes, if the EPUB has one. We pull the cover out of the book, re-encode it as a JPEG and write it into the file, so it turns up in your player's library.
Is M4B the same as M4A or AAC?
The audio inside is the same AAC either way. M4B is the audiobook extension of the same container, and the extension is what tells a player to treat the file as a book. We also set the media type flag inside the file, which is what Apple Books actually reads.
How big is the file?
About 29 MB an hour, so a ten-hour book comes out near 290 MB. That is 64 kbps mono, which is the right place for speech. Higher bitrates make the file bigger without making a narrator sound better.