M4B to Opus Converter
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Supports: M4B
Advanced Options - Our defaults are optimized for the best results. We recommend you keeping the defaults unless you have a specific need.
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Need help?How to Convert M4B to Opus Online
- Upload Your M4B File — Click "Choose Files" or drag and drop your .m4b audiobook. Batch conversion is supported.
- Set Audio Quality — Under "File Compression," choose "Quality Preset" (Highest → Lowest) or pick a specific bitrate:
- Constant Bitrate: 8 kbps – 384 kbps (default 128 kbps)
- Custom Bitrate: Enter any value in bps, Kbps, or Mbps (default 128 Kbps)
- Variable Bitrate: Opus ranges from 6k–24k (low) up to 320k–510k (max)
- Specific file size: Target an exact output size (default 8 MB)
- Set Audio Channel (Optional) — Keep original, force Mono, or force Stereo.
- Set Sample Rate (Optional) — Keep original or choose 8000, 12000, 16000, 24000, 44100, or 48000 Hz.
- Trim (Optional) — Set a start time and duration in seconds or HH:MM:SS.sss to extract a specific chapter or segment.
- Convert and Download — Click "Convert" and download your Opus file. No sign-up required.
Why Convert M4B to Opus?
- Constant Bitrate: 8 kbps – 384 kbps (default 128 kbps)
- Custom Bitrate: Enter any value in bps, Kbps, or Mbps (default 128 Kbps)
- Variable Bitrate: Opus ranges from 6k–24k (low) up to 320k–510k (max)
- Specific file size: Target an exact output size (default 8 MB)
M4B is Apple's audiobook format — essentially an M4A file with chapter bookmark support. Opus is a modern open-source codec that delivers better audio quality than MP3 or AAC at equivalent bitrates, and is the standard for WebRTC, Discord, and web audio streaming.
Converting M4B to Opus is useful for listening to audiobooks on non-Apple devices, reducing file size for mobile storage, or preparing audio for web-based players that support Opus natively.
Note: Chapter bookmarks from the M4B file are not preserved in Opus format. Use the Trim option to extract specific chapters by time range.
M4B vs Opus Comparison
| Feature | M4B (source) | Opus (output) |
|---|---|---|
| Chapter bookmarks | ✅ Yes | ❌ No |
| Quality at 64 kbps | Good (AAC) | Excellent |
| Apple device support | ✅ Native | ❌ Needs app |
| Web/Discord support | ❌ No | ✅ Native |
| File size (1 hr audiobook) | ~30 MB | ~15–20 MB |
Frequently Asked Questions
Will chapter bookmarks be preserved?
No. Opus doesn't support chapter metadata. Use the Trim option to extract individual chapters by setting start time and duration.
What bitrate should I use for audiobooks?
Audiobooks are speech, so 32–64 kbps Opus sounds excellent. For music-heavy audiobooks, use 96–128 kbps.
What's the advantage of Opus over MP3?
Opus delivers noticeably better quality at low bitrates (under 64 kbps) and has lower latency, making it ideal for speech content like audiobooks.
Can I batch convert?
Yes. Upload multiple M4B files and download Opus results individually or as a ZIP archive.