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? Email sebas.bur@gmail.com or open an issue on GitHub.