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
Title is required.
Author is required.
Verify metadata (optional)
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, which is 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.