mp3tom4b

Drag your audio files in. Get an M4B audiobook out. Your files never leave your browser.

A free, browser-based tool that converts MP3, M4A, WAV, FLAC, OGG, and Opus files (loose or in a ZIP) into M4B audiobook files with chapters and embedded cover art. All conversion happens client-side via WebAssembly — nothing is uploaded to any server.

🌐 Live: mp3tom4b.com

What makes this different

Other web converters (CloudConvert, Vertopal, Movavi, etc.) upload your files to their servers. mp3tom4b doesn't — the entire conversion runs inside your browser via ffmpeg.wasm. The repo is open source so you can verify that for yourself.

Verify it yourself

You don't have to take our word for it:

1. Open your browser's DevTools → Network tab.
2. Drop in some audio files and run a conversion.
3. Watch: there are no upload requests. The only network traffic is the one-time download of the ffmpeg WebAssembly core. Your audio never leaves the tab.

Features

• Drag-and-drop multi-file input — drop audio files and a cover image together, or a whole folder.
• Each file becomes a chapter, in your chosen order, with editable titles.
• Auto-fill of audiobook metadata from embedded tags, plus optional online lookup via iTunes / Open Library (only metadata leaves the browser, only on explicit click).
• Embedded cover art (JPG / PNG / WebP), auto-resized to 1200×1200.
• Bitrate options from 64 kbps (audiobook standard) up to 256 kbps, with a smart default and live output-size estimate.
• Estimated time remaining during conversion.
• Five SEO sister pages (/flac-to-m4b, /wav-to-m4b, /m4a-to-m4b, /ogg-to-m4b, /opus-to-m4b).
• Light + dark theming that follows your OS preference.
• 100% client-side — files never leave your browser.
• Free, no sign-up, no ads, no upload size limits.

See FEATURES.md for the full list.

See also: CHANGELOG.md, BACKLOG.md.

Tech Stack

• Next.js 14 + TypeScript + Tailwind CSS
• ffmpeg.wasm for in-browser audio encoding
• Hosted on Vercel

Local Development

npm install
npm run dev

Open http://localhost:3000.

Requires Node 20+.

License

MIT (see LICENSE).

Author

Built by Burcevski ICT in Amersfoort, the Netherlands.