MKV is the Matroska container, which will hold almost any codec, any number of tracks and any subtitle format you throw at it. Opus is the lossy encoding voice calls and browsers settled on, better than MP3 at the same bitrate and unreadable by most hardware players. ffmpeg sits between the two, and this page says exactly what happens to an MKV file on the way to becoming an OPUS one.
What runs when an MKV file becomes OPUS
ffmpeg held to a single thread, on a machine we rent and watch. Your MKV file is uploaded once, ffmpeg runs once, the OPUS comes back, and neither file is kept. MKV to OPUS is one of the 3649 pairs that engine was probed on with a real file, which is why it has a page here and why the pairs the probe could not prove do not.
What survives from the MKV into the OPUS
The sound, and nothing but the sound. ffmpeg passes -vn on every audio target, so the MKV video stream is dropped at the demultiplexer and never reaches an encoder at all. That is why this pair is quick: the picture is the expensive part of an MKV file, and this pass does not look at it.
The running time. The OPUS plays for exactly as long as the MKV did, because nothing on this path trims, cuts or speeds anything up.
The sample rate and the channel layout of the MKV audio track, carried into the OPUS unchanged wherever the encoder accepts them, and resampled only where it cannot.
What MKV to OPUS costs you
The picture, all of it, and on purpose. The MKV video stream is dropped before any encoder sees it, and the OPUS that comes back is sound alone. Nothing in the OPUS remembers there was ever an image, and no later conversion can put one back.
Cover art carried inside the MKV. ffmpeg counts an attached picture as a video stream, so the same -vn that drops the film drops the artwork with it. Keeping it is worse: on several audio containers the mux then fails outright with an error nobody can read.
Any second audio track, and the subtitles. ffmpeg selects one stream out of the MKV, and an OPUS file has nowhere to put a subtitle track in any case.
Exactness, one more time. The audio inside the MKV was compressed once already, and OPUS compresses the decoded result again. Two lossy passes do not cancel each other out, they stack.
If a model is going to read the OPUS
An OPUS file is a waveform, and a language model reads text, so handing an assistant this OPUS directly gets you nothing. The OPUS becomes useful the moment somebody turns the sound into words, and that is a separate job with its own page here. MKV to OPUS is the step before it: get the sound out of the MKV in a container the transcriber accepts, then hand the model a transcript instead of a file it cannot open.
Running MKV to OPUS against the real engine with a real file, ffmpeg wrote 30,246 bytes of OPUS in 0.1 seconds, machine otherwise idle. That is one MKV file on one day and not an average, which is why the number is given together with the file that produced it.
The MKV to OPUS verdict was reached by ffprobe, meaning the container was actually demultiplexed, recognised under the right name, and carried at least one stream. Which check was used matters, because they do not all prove the same thing, and a status code of 200 proves nothing whatsoever about whether the OPUS file has anything inside it.
The same MKV file, sent somewhere else
Other outputs the probe measured out of an MKV file, so the cost of choosing OPUS can be read against something. Sizes do not compare across engines, because each family was probed with its own MKV witness file.
MKV to GIF: 164,734 bytes in 0.1 seconds, verified by a format signature.
MKV to MP3: 24,677 bytes in 0.1 seconds, verified by ffprobe.
MKV to MP4: 46,085 bytes in 0.2 seconds, verified by ffprobe.
MKV to WAV: 265,422 bytes in 0.1 seconds, verified by ffprobe.
MKV to FLAC: 148,596 bytes in 0.1 seconds, verified by ffprobe.
MKV to OGG: 8,384 bytes in 0.1 seconds, verified by ffprobe.
Other ways into OPUS, and what they measured
Among the published routes into OPUS, MKV is the first largest output of the 16 measured. The witness files differ, so this ranks the probe run and not your document.
MOV to OPUS: 27,836 bytes in 0.2 seconds.
MPG to OPUS: 27,831 bytes in 0.2 seconds.
OGG to OPUS: 30,214 bytes in 0.2 seconds.
WAV to OPUS: 27,591 bytes in 0.2 seconds.
WEBM to OPUS: 26,907 bytes in 0.2 seconds.
What we will not pretend about MKV to OPUS
An MKV file over 150 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
An MKV to OPUS run that passes 240 seconds is killed, and the ffmpeg process is killed with it. A run left behind would sit on one of the machine's two cores until somebody noticed.
The MKV file behind the measurement on this page was 3 seconds of 320 by 240 test pattern at 25 frames a second, with a sine wave on the audio track, built by ffmpeg for the probe itself. That is why MKV to OPUS came out at 0.1 seconds, and it is not what your file will do. The measurement proves the pair works and says almost nothing about how long yours will take.
MKV to OPUS costs half a credit per conversion, whatever the length of the MKV. That is the same as an image or a document here, and it is a measurement rather than a gesture: an audio target on this engine is done in under two seconds, where a video target can take minutes.
An MKV longer than 60 minutes is refused before encoding. ffprobe measures the MKV first and answers with a refusal, rather than starting a job the 240 second guard would kill halfway to the OPUS.
MKV to OPUS waits at the audio door, which is 2 slots wide and gives up after 5 seconds rather than making you wait twenty times the length of the work. Until the 30th there was one door for everything, and a single video transcode starved every audio conversion for its whole duration: an OPUS that needed less than a second was answered with a refusal after twenty.
Nothing here reads what is inside the MKV. No transcription, no scene detection, no summary: MKV to OPUS is a container and codec operation, and the OPUS says exactly what the MKV said, in a different wrapper. The sound is the whole payload.
One MKV file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
MKV to OPUS: what people ask
What actually converts my MKV file to OPUS?
ffmpeg does it, held to a single thread, on a machine we rent and watch. Not a browser trick and not somebody else service: the MKV file is uploaded once, ffmpeg runs once, the OPUS comes back, and neither file is kept afterwards.
How long does MKV to OPUS take?
The probe measured 0.1 seconds for 30,246 bytes of OPUS, on a 3 second file. An audio target scales far better than a video one: three minutes of source through this engine takes under a second, because only the sound is encoded. Past 240 seconds any run is stopped, and an audio one does not get close.
What do I lose going from MKV to OPUS?
The one to know about first: The picture, all of it, and on purpose. The MKV video stream is dropped before any encoder sees it, and the OPUS that comes back is sound alone. Nothing in the OPUS remembers there was ever an image, and no later conversion can put one back.
Does the OPUS still have the video in it?
No, and that is the entire point of the pair. The MKV picture is dropped before any encoder touches it, so what you download is sound only. If you wanted to keep the picture you wanted a video target, and it is priced differently because it costs differently.
Is MKV to OPUS free?
There is a free allowance every month, and one MKV to OPUS conversion costs half a credit against it however long the MKV runs. An audio target is charged per conversion and not per minute, because the engine encodes three minutes of sound in well under a second.