Convert MP3 to M4B for Android

Android audiobook players read M4B chapters, cover and metadata straight from the file, so one .m4b per book is the simplest thing to carry. Kataribe builds it from your MP3s in the browser on your computer; then you copy one file to the phone.

The typical case: a shelf of DRM-free audiobooks bought as MP3 zips, each a folder of 20 to 90 files. Players like Smart AudioBook Player cope with folders, but every book means copying a folder, hoping the order survives, and living with chapter names like "Track 12". Drop a book's files here; the ID3 tags fill in title, author, narrator and cover, track numbers set the order, and each file becomes a chapter you can rename. Build, then copy the .m4b to the phone by USB, cloud drive or SD card.

Do the conversion on a laptop or desktop rather than in the phone's browser. Mobile browsers aren't recommended for long books: the build is a long job that needs the tab kept open and more memory than phone browsers tend to allow. The FFmpeg engine (about 30 MB) downloads once and is cached, and the files never leave your machine.

After you download the .m4b

Android players

  1. Copy the .m4b to the phone (USB, Drive, or Syncthing) into a folder your player watches.
  2. Smart AudioBook Player, Listen, Voice, and the Audiobookshelf app all read M4B chapters and cover art.
  3. One file per book means one entry in the library, with your position remembered across chapters.

Frequently asked questions

Which Android apps read the chapters?
Smart AudioBook Player and VLC play M4B, as does the Audiobookshelf app if you run a server. The chapters are standard MP4 markers, so any Android player that lists M4B support reads the same ones.
Does the M4B remember my place?
The player does that per book, and with one file per book there's nothing to confuse it. Folder-based books lose their place more easily when files are re-copied or renamed.
How do I get the file onto the phone?
USB in file-transfer mode, a cloud drive, or an SD card. Put it in the folder your player scans; Smart AudioBook Player asks for a root audiobooks folder and lists each M4B inside it as a book.
Can I run this in Chrome on the phone instead?
Not recommended for a full-length book. The engine runs in mobile browsers, but a long encode needs the tab kept open and more memory than phones usually allow. Build on a computer and copy the file over.
Are my audio files uploaded anywhere?
No. Kataribe runs FFmpeg compiled to WebAssembly inside your browser, so the files never leave your device. The engine (about 30 MB) downloads once and is cached; after that there is no account, no queue and no watermark.
How long does a full book take?
Encoding runs many times faster than real time on a modern laptop, but a full-length book still takes minutes, not seconds. Keep the tab open until the download appears. If every input is already AAC, the audio is copied rather than re-encoded, which is much quicker.
Is this really free?
Yes. Every input format, every bitrate, cover art and metadata, no sign-up, and nothing added to the audio.
Does it work with DRM-protected audiobooks?
No. DRM-protected files (Audible .aax and similar) won't open, and we don't help with that. Kataribe is for audio you already have in plain MP3, M4A, WAV, FLAC or OGG.