FLV is the Flash Video container, orphaned the day the plugin that gave it a name was switched off. 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 FLV file on the way to becoming an MKV one.
What runs when an FLV file becomes MKV
ffmpeg held to a single thread, on a machine we rent and watch. Your FLV file is uploaded once, ffmpeg runs once, the MKV comes back, and neither file is kept. FLV 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 FLV into the MKV
The aspect ratio of the FLV. 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 FLV 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 FLV to an audio format, where the picture is thrown away instead.
The running time and the order of the frames. ffmpeg re-encodes the FLV, it does not re-cut it, so the MKV starts and ends where the FLV did.
What FLV to MKV costs you
Every pixel, one more time. The FLV is decoded to raw frames and re-encoded as MKV, so whatever the FLV 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 FLV 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 FLV. ffprobe measures the duration before the encoder starts, and a longer FLV 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 FLV 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 FLV, 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 FLV to MKV against the real engine with a real file, ffmpeg wrote 28,694 bytes of MKV in 0.2 seconds, machine otherwise idle. That is one FLV file on one day and not an average, which is why the number is given together with the file that produced it.
The FLV 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 FLV file, sent somewhere else
Other outputs the probe measured out of an FLV 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 FLV witness file.
FLV to GIF: 173,530 bytes in 0.1 seconds, verified by a format signature.
FLV to MP3: 24,677 bytes in 0.1 seconds, verified by ffprobe.
FLV to MP4: 48,269 bytes in 0.3 seconds, verified by ffprobe.
FLV to WAV: 267,342 bytes in 0.1 seconds, verified by ffprobe.
FLV to FLAC: 156,775 bytes in 0.1 seconds, verified by ffprobe.
FLV to OGG: 9,138 bytes in 0.2 seconds, verified by ffprobe.
Other ways into MKV, and what they measured
Among the published routes into MKV, FLV is the fifth largest output of the 9 measured. The witness files differ, so this ranks the probe run and not your document.
GIF to MKV: 54,065 bytes in 0.3 seconds.
MOV to MKV: 26,416 bytes in 0.2 seconds.
MPG to MKV: 34,577 bytes in 0.2 seconds.
WEBM to MKV: 28,106 bytes in 0.3 seconds.
WMV to MKV: 28,796 bytes in 0.2 seconds.
What we will not pretend about FLV to MKV
An FLV file over 150 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
An FLV 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 FLV 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 FLV 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 FLV running time and twice it, not the tenth of a second the probe file took.
An FLV 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.
FLV to MKV is the single conversion here that is not charged at the floor rate. It costs 2 credits per minute of FLV, every started minute counts, and because the engine refuses past 120 seconds one FLV 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. FLV 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 FLV. No transcription, no scene detection, no summary: FLV to MKV is a container and codec operation, and the MKV says exactly what the FLV said, in a different wrapper. The picture is the whole payload.
One FLV file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
FLV to MKV: what people ask
What actually converts my FLV 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 FLV file is uploaded once, ffmpeg runs once, the MKV comes back, and neither file is kept afterwards.
How long does FLV to MKV take?
The probe measured 0.2 seconds for 28,694 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 FLV running time and twice it. The FLV is refused past 120 seconds, so the worst case is bounded.
What do I lose going from FLV to MKV?
The one to know about first: Every pixel, one more time. The FLV is decoded to raw frames and re-encoded as MKV, so whatever the FLV 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 FLV?
Because the engine caps every video output at 1280 pixels wide. Encoder cost grows with pixel count, and a 4K FLV would cost about sixteen times a 1080p one on a machine that also serves everybody else. An FLV at or below 1280 pixels comes back at its own size, untouched.
Is FLV to MKV free?
There is a free allowance every month, and FLV to MKV is the one conversion here that is not charged at the floor rate: 2 credits per minute of FLV, 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.