What is an OPUS file?

OPUS is a codec: the encoding itself rather than the box it travels in. It is used for streaming and the web.

The extension is .opus, and the full name is Opus Audio. Both matter less than what the file can hold, which is what the rest of this page is about.

Xiph.Org published it in 2012. The specification is RFC 6716.

Age is worth knowing here for one practical reason: the older a format is, the more programs have had time to learn it.

It is published in full, so anyone can implement it from the document rather than by inspection, which is why it turns up in so many programs and why files written twenty years ago still open. A published specification is not the same thing as a royalty-free one: where a format wraps a codec, the patent licensing is a separate question the standard does not settle.

OPUS discards detail to stay small, and what it discards does not come back. That is the right trade for something being looked at and the wrong one for something being edited: each re-save takes another bite.

OPUS can hold a layout that lets it start playing before it has finished arriving.

That matters mostly when converting: whatever the target cannot hold is dropped, usually without a warning.

Up to 255. Two covers stereo and is what almost everything is; more than two is surround, and it is the part most likely to be folded down when converting to a format that does not define it.

VLC, FFmpeg and Audacity read it, and so do most programs of the same kind.

If a file will not open, the format is rarely the problem — it is more often that the program predates it. Converting to something older is the reliable way past that, and it is what the rest of this site is for.

Current browsers read it; older ones do not.

Where support is uneven, serving a second format alongside it is the usual answer — the browser takes the one it understands and nobody is left out.

OPUS is meant to be handed over rather than worked in. Editing one is possible and rarely pleasant; the sane approach is to change the source and export again.

The recurring complaints: support outside its home ground is uneven.

None of these is a reason to avoid the format. They are the things worth knowing before you are surprised by one, which is a different claim and a more useful one.

WhatsApp, Telegram, Signal and Discord all encode voice messages as Opus. Nothing in the interface mentions it, so the format only becomes visible at the moment somebody exports a chat, saves an attachment, or tries to open the file on a desktop and finds that nothing will.

That is the situation nearly everyone here is in. The file is a recording of someone talking — often something worth keeping, sometimes something that matters legally — and the question is how to get it into a shape that ordinary software will accept.

This is not promotion. Opus was standardised by the IETF in 2012 as RFC 6716, is royalty-free, and in listening tests beats MP3, AAC and Vorbis at essentially every bitrate. At 64 kbps it beats the others on stereo music rather than sounding identical to the source; Xiph puts that point near 128. For speech it is transparent far lower.

It manages that by combining two designs — one built for speech, one for music — and switching between them, or blending them, according to what it hears. Where MP3 was designed for music and copes with speech, Opus was designed for both from the start.

Opus was designed for real-time audio, so its latency is very low — a frame can be as short as 2.5 milliseconds, against over 20 for MP3. In a call that is the difference between people talking naturally and talking over each other.

It also survives a poor connection. The encoder can change bitrate mid-stream as the network varies, and a lost packet degrades the sound briefly rather than producing a gap or a click. This is why it is the codec inside WebRTC, and therefore inside effectively every browser-based call and voice chat.

A minute of voice message is often under 100 kilobytes. That is not compression at the cost of intelligibility — it is a codec designed for speech running at a bitrate suited to speech, typically somewhere between 16 and 32 kbps for a mono voice recording.

Converting to MP3 will usually make the file several times larger while sounding no better, and often slightly worse. That surprises people who assume MP3 is the more capable format. It is the more compatible format, which is a different thing and the only reason to convert.

Opus audio turns up as `.opus`, which is the codec in an Ogg container, and as `.ogg`, which is the same container possibly holding Opus and possibly holding Vorbis. It also appears inside WebM and inside MKV, where it has no distinct extension at all.

WhatsApp adds its own wrinkle by saving voice notes with a `.opus` extension on some platforms and inside an MP4 container on others. If a file will not open in something that claims Opus support, a mismatch of this kind is the usual cause rather than damage.

A specific and shrinking list. Older versions of Windows Media Player and QuickTime. Most car stereos. Many older phones. A good number of upload forms and transcription services that check the extension against a list written before 2015. Some audio editing software still treats it as exotic.

Everything else copes: every current browser, VLC, Android since version 5, modern Windows and macOS, and most media players. Before converting, it is worth confirming the destination is really on the short list — a great deal of unnecessary conversion happens on the assumption that it is.

Opus to MP3 is lossy to lossy. The MP3 encoder receives audio that has already had material removed and removes more, working from a model that does not know what the first pass discarded. For a voice recording the damage is generally inaudible; for music it is more noticeable, particularly if the source was already at a low bitrate.

That does not make it wrong. A slightly degraded file that plays in the car beats a perfect one that does not. Use 128 kbps or higher for speech and keep the original Opus, which stays the better copy and takes almost no space.

Convert to MP3 when the destination is old — a car, a legacy player, a submission form. Convert to WAV when a transcription tool, a speech pipeline or an audio editor demands uncompressed input, which many do because reading raw samples removes a decoder from their problem.

Do not convert at all if the file is going onto a phone, a computer or a website, or into an archive. Opus is smaller and better; the only argument against it is compatibility, and where compatibility is not in question there is nothing to gain.

Think about what is actually in these files. Messages from family, medical discussions, arguments, negotiations, recordings kept because somebody might need to prove what was said. Uploading one to a website in exchange for a download link is a poor trade that people make routinely for want of an alternative.

The conversion here happens inside the browser tab, so the recording never leaves the device. There is no upload, no queue, no retention period to read about, and no daily allowance being spent, because none of it touches a server of ours.

The limits that do apply are the free ones: 100 MB per file, and 100 files in a batch. Individual Opus voice notes are far too small for the first to matter — the codec is built for speech at a few dozen kilobits — so the one to plan around is the second, which is the case when the export is an entire chat history rather than one message. Drop a folder of a thousand notes and only the first hundred are converted.