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.

How to convert MP3 to M4B for Android players

  1. Build on a computer, not on the phone. Open the builder in a desktop browser and drop the book's MP3s. Order follows track tags when every file has one, natural filename sort otherwise, and the arrows on each row fix exceptions. The phone only receives the finished file.
  2. Keep chapter titles short. Android players show the chapter list in a portrait column, so long ID3 titles get truncated. Defaults are the ID3 title or the cleaned filename; trim "The Long Road Home, Chapter Twelve: The Return" to "12. The Return" here and it reads cleanly in the player.
  3. Choose the bitrate for phone storage. 64 kbps AAC is around 29 MB an hour, so a 15-hour novel is 430 MB and a shelf of twenty such books is 8.6 GB. If the phone is tight, 48 kbps saves a quarter of that with a mild loss on speech; 32 is audibly worse. Mono sources stay mono, which is what keeps the numbers low.
  4. Embed a cover the player will show. Smart AudioBook Player and Voice display the embedded art in the library and while playing; nothing else is needed. A square JPEG of 600 to 1000 pixels is plenty for a phone screen; the image goes in as-is, so a print-resolution scan just adds megabytes.
  5. Set author and title, then build and copy. The download is named Author - Title.m4b, which is a sensible name in a player's file list. Copy it into the folder your player scans; the transfer options and the scan are covered below.

Android players that honor M4B chapters

Smart AudioBook Player, Voice, Listen and the Audiobookshelf app all read the MP4 chapter list and expose it as a chapter menu with skip buttons; VLC does too, from its playback menu. Each keeps its own position per book, so the M4B file itself is portable between them.

What differs is how they find books. Smart AudioBook Player and Voice scan a root folder you choose and treat each file or subfolder as a book; the Audiobookshelf app streams or downloads from your server. In every case one M4B is one book, which is why merging matters more on Android than the container does.

Folder books versus one file per book

Folder-based players do work with the original MP3 folders, and for a book you'll listen to once that is fine. The problems are operational: copying 90 files over MTP is slow and occasionally drops one, renaming a folder can reset the position in some apps, and the chapter names are whatever the ripper wrote.

One M4B per book avoids each of those. A single file transfers atomically, position is keyed to the file, the chapter names are the ones you typed, and the cover is inside. It also survives being moved to an SD card, a cloud folder or a new phone as one object.

Bluetooth, speed and the things the M4B doesn't change

Playback speed, skip-silence, sleep timers and headphone-button behavior are player features and work the same for MP3 folders and M4Bs. What the file format changes is structure: chapters, one position, one cover, one entry. Don't expect the conversion to fix a quiet narrator or a noisy recording; the audio is re-encoded, not processed.

Bitrate doesn't interact with Bluetooth either. The phone decodes the AAC and re-encodes for the headset with whatever codec the two negotiate, so for narration the difference between a 64 and a 128 kbps book is hard to hear through the headset's own codec. Spend the storage elsewhere.

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.
Why does my audiobook show up in a music app on Android?
Android's media scanner indexes any MP4/AAC file, and music apps list whatever the scanner finds. The M4B carries the audiobook media type, but not every music app honors it. Put the file in a folder your audiobook player owns and, if the music app respects it, add a .nomedia file to that folder.
Can I use Audible or Google Play Books to play my own M4B?
No. Neither app imports audio files you supply. Use a player that scans local storage: Smart AudioBook Player, Voice, Listen, or the Audiobookshelf app with your own server; all of them list M4B chapters.
Will the same M4B also work on an iPhone?
Yes. It is one MP4 file with AAC audio; the Android player and Apple Books read the same chapters, cover and tags. Build once and copy it to both devices.
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.