Opus to M4B
Convert Opus audio files into a chaptered M4B audiobook, right in your browser. Great for podcast recordings, internet radio archives, and open-source narrations. Your files never leave your device.
Drop audio files + cover image, or click to browse
MP3 · M4A · WAV · FLAC · OGG · Opus · JPG · PNG
New here? M4B is the audiobook format Apple Books, Plex, and most modern players recognise: chapters, cover art, and resume position in one tagged file. Drop your audio in and you'll get back a single M4B with chapters at every file boundary.
- One chapter per file, titled from the filename
- Optional cover art, embedded at 1200×1200
- More detail on the FAQ page
Audiobook details
Verify metadata (optional)
Enter a title, author, or narrator first.
Cover image (optional)
Drop or click to add cover
JPG · PNG
Square images work best. Non-square covers are padded to 1200×1200 with a white background and embedded as JPEG.
Output bitrate
96 kbps, 128 kbps, 192 kbps, 256 kbpsTo enable: add at least one audio file, enter a title, enter an author.
How it works
- Step 1
Drop your files
Drag MP3, M4A, WAV, FLAC, OGG, or Opus files into the browser. They stay on your device.
- Step 2
Add metadata
Reorder chapters, set the title, author, narrator and cover. Embedded tags auto-fill what they can.
- Step 3
Download M4B
A single tagged, chaptered audiobook, ready for Apple Books, Plex, or any M4B-aware player.
Opus to M4B: common questions
- Where do Opus audiobooks come from?
- Opus is a modern open codec developed by Xiph.Org and standardised by the IETF. It is used by some podcast apps, internet radio tools, and open-source recording software. If you recorded narration with Audacity or a similar tool and exported to Opus, or downloaded content from a service that uses Opus, mp3tom4b can package those files into a standard M4B.
- Is Opus better quality than AAC at the same bitrate?
- For low-bitrate speech, Opus is often slightly better than AAC, but at 64 kbps (the standard for audiobooks) both codecs are transparent for spoken audio. The M4B format requires AAC, so mp3tom4b re-encodes Opus to AAC during conversion. The audible difference is negligible.
- My Opus files have a .ogg extension. Will they work?
- Yes. Opus audio is often stored in an OGG container with a .ogg or .opus extension. mp3tom4b detects the codec from the file content, not just the extension, so files named .ogg that contain Opus audio will be handled correctly.
- Are my Opus files uploaded anywhere?
- No. Conversion runs entirely in your browser via a WebAssembly build of FFmpeg. Your audio files never leave your device. Open the Network tab in developer tools while converting to confirm; there are no upload requests.