About mp3tom4b

mp3tom4b is a free, open-source tool that turns a folder of audio files into a single chaptered M4B audiobook, without uploading anything.

How it works

The conversion happens inside your browser via a WebAssembly build of FFmpeg. When you drop files in, they are read directly from your disk by your browser, processed locally, and the finished M4B is offered as a download. No bytes touch any server we operate.

You can verify this by opening your browser's network tab while converting; you'll see no upload traffic. The full source code is published on GitHub so you can audit it yourself.

Why it exists

Every other browser-based M4B converter we could find uploads your audio to a server. Local tools exist (ChapterForge, m4b-tool, AudioBookConverter), but they require installation, and most either cost money or assume you're comfortable on the command line. There was a gap for a fast, private, no-install option. So we built one.

Who built it

mp3tom4b is built and maintained by Burcevski ICT in Amersfoort, the Netherlands. Questions or feedback? Open an issue on GitHub or email sebas.bur@gmail.com.