M4B → OPUS · Free · No Upload
Free Online M4B to OPUS Converter — No Installation
Convert M4B (audiobook) to Opus right in your browser, in seconds. Free, unlimited, and private: your file never leaves your device.
100% local processing: your file never leaves your device. Once the engine is ready, this tab can keep converting offline. Reopening or reloading offline may not work.
Supported formats:Files .m4b up to 300 MB — processed locally · Video and virtually any audio format are also accepted — we extract the audio automatically.
How it works
- 01
Upload the M4B file
Drag the .m4b file to the area above or click to select. Files up to 300 MB are accepted — everything is processed locally.
- 02
The conversion runs in your browser
FFmpeg (the same engine used by professionals) runs in WebAssembly right on the page. No bytes are sent to servers — total privacy.
- 03
Download the OPUS
In seconds, the .opus file is ready for download, in Opus 96 kbps — the best quality per MB among modern codecs.
Frequently asked questions
Why convert M4B to OPUS?
M4B features (chapters, resume-where-you-left-off) only work in Apple apps and dedicated readers — many players won't even open the file. Converting to MP3 makes the audiobook play on any device; chapters become one continuous audio track. Opus is the best quality-per-megabyte codec available today: perfectly intelligible speech at 16–24 kbps.
Where do M4B files come from?
Typically from audiobooks from iTunes/Apple Books, audiobook assembly tools (m4b-tool, Audiobook Builder) and downloaded audiobook libraries. Same MP4 container as M4A; the .m4b extension tells the player to treat it as an audiobook (bookmarks). Files easily exceed 10 hours — watch the size when converting.
Is it worth using OPUS?
Opus is the best quality-per-megabyte codec available today: perfectly intelligible speech at 16–24 kbps. It's the standard for WhatsApp and Discord. Standardized by the IETF in RFC 6716 (2012), combining SILK (Skype) and CELT (Xiph); it always operates internally at 48 kHz, with latency low enough for calls.
Do I lose quality when converting M4B to OPUS?
There is a re-encoding between two compressed formats, with minimal technical loss — imperceptible for voice and most uses. If you're going to edit the audio multiple times, prefer converting to WAV or FLAC and compress only at the end.
What if I need a transcript of this audio?
After converting, you can transcribe the file with AI on VoxScriber: 30 free minutes per month, with automatic punctuation and speaker identification. Ideal for WhatsApp audio messages, meetings, and interviews.
Is my file sent to any server?
No. All processing is local (FFmpeg compiled to WebAssembly running on the page). The file never leaves your device—it even works offline after the page loads.
Is there a size or quantity limit?
The limit is 300 MB per file (browser memory). There is no limit on the number of uses. For larger files, VoxScriber processes them on the server with uploads up to 5 GB.
Need the text from this audio? Transcribe with AI — 30 min free
Create free account →30 minutes/month free. No credit card required.
Keep exploring
Tools
Convert M4B to MP3
MP3 is the format that opens on ANY device made since the 1990s — the safe choice for sharing, attaching to email, and playing on your car stereo.
Convert M4B to WAV
WAV is the WORKING format: lossless editing, accepted by every audio software, and what telephony/IVR systems usually require (8 kHz mono WAV).
Convert M4B to M4A
AAC delivers the same quality as MP3 with about 30% less space — it's MP3's technical successor and the Apple ecosystem standard.
Convert MP3 to OPUS
Leaving MP3 usually means needing WAV to edit without accumulating loss (recompressing MP3 degrades every generation) or a format required by a specific system — IVR/telephony wants 8 kHz WAV, iPhone ringtones want M4R.
Convert WAV to OPUS
WAV takes about 10 MB per minute (16-bit/44.
Convert M4A to OPUS
Older stereos, car radios, and some corporate systems won't open M4A — an iPhone voice memo arrives and no one can play it.