MKV is the Matroska container, which will hold almost any codec, any number of tracks and any subtitle format you throw at it. 3GP is the video container of phones from before they recorded MP4. ffmpeg sits between the two, and this page says exactly what happens to an MKV file on the way to becoming a 3GP one.
What runs when an MKV file becomes 3GP
ffmpeg held to a single thread, on a machine we rent and watch. Your MKV file is uploaded once, ffmpeg runs once, the 3GP comes back, and neither file is kept. MKV to 3GP 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 3GP
The aspect ratio of the MKV. 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 MKV already 1280 pixels wide or narrower is not resized at all.
The audio track, which travels into the 3GP alongside the picture. That is the whole difference between this pair and sending the same MKV to an audio format, where the picture is thrown away instead.
The running time and the order of the frames. ffmpeg re-encodes the MKV, it does not re-cut it, so the 3GP starts and ends where the MKV did.
What MKV to 3GP costs you
Every pixel, one more time. The MKV is decoded to raw frames and re-encoded as 3GP, so whatever the MKV 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 MKV down to 1280 because encoder cost grows with pixel count: a 4K source costs about sixteen times a 1080p one, for a 3GP 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 3GP, 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 MKV. ffprobe measures the duration before the encoder starts, and a longer MKV comes back as a refusal in under a second rather than as a timeout four minutes later.
Where MKV and 3GP files come from
3GP. The 3GPP container, made for phones on 3G networks: small frames, low bitrates, and the format of every video shot before about 2012. It is MP4 underneath with tighter constraints.
If a model is going to read the 3GP
Nothing about MKV to 3GP helps a language model, and pretending otherwise would be a sales line rather than a fact. A 3GP is pixels and sound, both expensive to read and neither of them text. If what you actually want is what is said in the MKV, the useful move is not 3GP 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 MKV to 3GP against the real engine with a real file, ffmpeg wrote 45,947 bytes of 3GP in 0.2 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 3GP 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 3GP 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 3GP 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 3GP, and what they measured
Among the published routes into 3GP, MKV is the seventh largest output of the 9 measured. The witness files differ, so this ranks the probe run and not your document.
MOV to 3GP: 45,545 bytes in 0.3 seconds.
MPG to 3GP: 49,242 bytes in 0.3 seconds.
WEBM to 3GP: 46,716 bytes in 0.3 seconds.
WMV to 3GP: 48,931 bytes in 0.2 seconds.
MP4 to 3GP: 45,545 bytes in 0.3 seconds.
What we will not pretend about MKV to 3GP
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 3GP 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 3GP 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 a 3GP to cost somewhere between a fraction of the MKV running time and twice it, not the tenth of a second the probe file took.
An MKV 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 3GP encoding is roughly two minutes of the only machine there is, and the live traffic shares the same two cores.
MKV to 3GP is the single conversion here that is not charged at the floor rate. It costs 2 credits per minute of MKV, every started minute counts, and because the engine refuses past 120 seconds one MKV to 3GP run can never cost more than 4 credits. The figure is on the button before you press it.
One 3GP 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. MKV to 3GP therefore queues on a single slot and gives up after 25 seconds rather than piling up.
Nothing here reads what is inside the MKV. No transcription, no scene detection, no summary: MKV to 3GP is a container and codec operation, and the 3GP says exactly what the MKV said, in a different wrapper. The picture 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 3GP: what people ask
What actually converts my MKV file to 3GP?
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 3GP comes back, and neither file is kept afterwards.
How long does MKV to 3GP take?
The probe measured 0.2 seconds for 45,947 bytes of 3GP, 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 MKV running time and twice it. The MKV is refused past 120 seconds, so the worst case is bounded.
What do I lose going from MKV to 3GP?
The one to know about first: Every pixel, one more time. The MKV is decoded to raw frames and re-encoded as 3GP, so whatever the MKV 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 3GP come back narrower than the MKV?
Because the engine caps every video output at 1280 pixels wide. Encoder cost grows with pixel count, and a 4K MKV would cost about sixteen times a 1080p one on a machine that also serves everybody else. An MKV at or below 1280 pixels comes back at its own size, untouched.
Is MKV to 3GP free?
There is a free allowance every month, and MKV to 3GP is the one conversion here that is not charged at the floor rate: 2 credits per minute of MKV, every started minute counted, capped at 4 credits because the engine refuses past 120 seconds. Re-encoding a 3GP costs real machine time, so it is priced on the time it costs. The number is shown before you press anything.