MP4 is the MPEG-4 container, which is what a phone records, what a browser plays, and what almost every player opens without being asked twice. MKV is the Matroska container, which will hold almost any codec, any number of tracks and any subtitle format you throw at it. ffmpeg sits between the two, and this page says exactly what happens to an MP4 file on the way to becoming an MKV one.
What runs when an MP4 file becomes MKV
ffmpeg held to a single thread, on a machine we rent and watch. Your MP4 file is uploaded once, ffmpeg runs once, the MKV comes back, and neither file is kept. MP4 to MKV 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 MP4 into the MKV
The aspect ratio of the MP4. The engine scales to the smaller of 1280 pixels and the source width, and lets the height follow, rounded to an even number because encoders working in 4:2:0 refuse an odd one. An MP4 already 1280 pixels wide or narrower is not resized at all.
The audio track, which travels into the MKV alongside the picture. That is the whole difference between this pair and sending the same MP4 to an audio format, where the picture is thrown away instead.
The running time and the order of the frames. ffmpeg re-encodes the MP4, it does not re-cut it, so the MKV starts and ends where the MP4 did.
What MP4 to MKV costs you
Every pixel, one more time. The MP4 is decoded to raw frames and re-encoded as MKV, so whatever the MP4 had already lost stays lost and this pass adds its own on top. Run one file through the loop four or five times and the damage is visible without looking for it.
Any width past 1280 pixels. The engine scales the MP4 down to 1280 because encoder cost grows with pixel count: a 4K source costs about sixteen times a 1080p one, for an MKV nobody asked to be that big. Below that width nothing is touched.
Subtitle and data tracks. ffmpeg passes -sn and -dn on the way into the MKV, because most of them do not fit the container and a mux that fails on a subtitle would waste a picture that converted perfectly well.
Anything past 120 seconds of MP4. ffprobe measures the duration before the encoder starts, and a longer MP4 comes back as a refusal in under a second rather than as a timeout four minutes later.
If a model is going to read the MKV
Nothing about MP4 to MKV helps a language model, and pretending otherwise would be a sales line rather than a fact. An MKV is pixels and sound, both expensive to read and neither of them text. If what you actually want is what is said in the MP4, the useful move is not MKV at all: pull the audio out as MP3, which costs half a credit and comes back in about a second, and have that transcribed.
Running MP4 to MKV against the real engine with a real file, ffmpeg wrote 26,436 bytes of MKV in 0.2 seconds, machine otherwise idle. That is one MP4 file on one day and not an average, which is why the number is given together with the file that produced it.
The MP4 to MKV 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 MKV file has anything inside it.
The same MP4 file, sent somewhere else
Other outputs the probe measured out of an MP4 file, so the cost of choosing MKV can be read against something. Sizes do not compare across engines, because each family was probed with its own MP4 witness file.
MP4 to GIF: 164,004 bytes in 0.2 seconds, verified by a format signature.
MP4 to MP3: 24,780 bytes in 0.2 seconds, verified by ffprobe.
MP4 to WAV: 266,318 bytes in 0.1 seconds, verified by ffprobe.
MP4 to FLAC: 156,949 bytes in 0.1 seconds, verified by ffprobe.
MP4 to OGG: 9,186 bytes in 0.1 seconds, verified by ffprobe.
MP4 to M4A: 27,723 bytes in 0.2 seconds, verified by ffprobe.
Other ways into MKV, and what they measured
Among the published routes into MKV, MP4 is the seventh largest output of the 9 measured. The witness files differ, so this ranks the probe run and not your document.
3GP to MKV: 26,436 bytes in 0.3 seconds.
AVI to MKV: 28,767 bytes in 0.2 seconds.
FLV to MKV: 28,694 bytes in 0.2 seconds.
GIF to MKV: 54,065 bytes in 0.3 seconds.
MOV to MKV: 26,416 bytes in 0.2 seconds.
What we will not pretend about MP4 to MKV
An MP4 file over 150 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
An MP4 to MKV 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 MP4 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 MP4 to MKV came out at 0.2 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.
Video is the one thing on this site that genuinely costs machine time, and the cost swings by two orders of magnitude with the encoder the container asks for: on this machine one minute of 720p source came out as AVI in 8.4 seconds and as WebM in 87, and the same minute in 1080p took 104. Expect an MKV to cost somewhere between a fraction of the MP4 running time and twice it, not the tenth of a second the probe file took.
An MP4 longer than 120 seconds is refused before any encoding starts, with an answer that names both limits, 150 MB and 120 seconds. It is not an upsell: two minutes of MKV encoding is roughly two minutes of the only machine there is, and the live traffic shares the same two cores.
MP4 to MKV is the single conversion here that is not charged at the floor rate. It costs 2 credits per minute of MP4, every started minute counts, and because the engine refuses past 120 seconds one MP4 to MKV run can never cost more than 4 credits. The figure is on the button before you press it.
One MKV encode at a time, and that was measured before it was decided. Two simultaneous transcodes do fit in memory, about 320 MB each, but each one goes from 104 seconds to 189: this machine is short of processor, not of memory, so throughput gains ten percent while the wait doubles. MP4 to MKV therefore queues on a single slot and gives up after 25 seconds rather than piling up.
Nothing here reads what is inside the MP4. No transcription, no scene detection, no summary: MP4 to MKV is a container and codec operation, and the MKV says exactly what the MP4 said, in a different wrapper. The picture is the whole payload.
One MP4 file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
MP4 to MKV: what people ask
What actually converts my MP4 file to MKV?
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 MP4 file is uploaded once, ffmpeg runs once, the MKV comes back, and neither file is kept afterwards.
How long does MP4 to MKV take?
The probe measured 0.2 seconds for 26,436 bytes of MKV, on a 3 second test pattern, and that number is honest about the plumbing and useless about your file. A video target re-encodes every frame: budget somewhere between a fraction of the MP4 running time and twice it. The MP4 is refused past 120 seconds, so the worst case is bounded.
What do I lose going from MP4 to MKV?
The one to know about first: Every pixel, one more time. The MP4 is decoded to raw frames and re-encoded as MKV, so whatever the MP4 had already lost stays lost and this pass adds its own on top. Run one file through the loop four or five times and the damage is visible without looking for it.
Why did my MKV come back narrower than the MP4?
Because the engine caps every video output at 1280 pixels wide. Encoder cost grows with pixel count, and a 4K MP4 would cost about sixteen times a 1080p one on a machine that also serves everybody else. An MP4 at or below 1280 pixels comes back at its own size, untouched.
Is MP4 to MKV free?
There is a free allowance every month, and MP4 to MKV is the one conversion here that is not charged at the floor rate: 2 credits per minute of MP4, every started minute counted, capped at 4 credits because the engine refuses past 120 seconds. Re-encoding an MKV costs real machine time, so it is priced on the time it costs. The number is shown before you press anything.