3GP is the video container of phones from before they recorded MP4. 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 a 3GP file on the way to becoming an MKV one.
What runs when a 3GP file becomes MKV
ffmpeg held to a single thread, on a machine we rent and watch. Your 3GP file is uploaded once, ffmpeg runs once, the MKV comes back, and neither file is kept. 3GP 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 3GP into the MKV
The aspect ratio of the 3GP. 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. A 3GP 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 3GP to an audio format, where the picture is thrown away instead.
The running time and the order of the frames. ffmpeg re-encodes the 3GP, it does not re-cut it, so the MKV starts and ends where the 3GP did.
What 3GP to MKV costs you
Every pixel, one more time. The 3GP is decoded to raw frames and re-encoded as MKV, so whatever the 3GP 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 3GP 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 3GP. ffprobe measures the duration before the encoder starts, and a longer 3GP comes back as a refusal in under a second rather than as a timeout four minutes later.
Where 3GP and MKV 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 MKV
Nothing about 3GP 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 3GP, 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 3GP to MKV against the real engine with a real file, ffmpeg wrote 26,436 bytes of MKV in 0.3 seconds, machine otherwise idle. That is one 3GP file on one day and not an average, which is why the number is given together with the file that produced it.
The 3GP 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 3GP file, sent somewhere else
Other outputs the probe measured out of a 3GP 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 3GP witness file.
3GP to GIF: 164,004 bytes in 0.1 seconds, verified by a format signature.
3GP to MP3: 24,780 bytes in 0.1 seconds, verified by ffprobe.
3GP to MP4: 45,683 bytes in 0.3 seconds, verified by ffprobe.
3GP to WAV: 266,318 bytes in 0.1 seconds, verified by ffprobe.
3GP to FLAC: 156,949 bytes in 0.1 seconds, verified by ffprobe.
3GP to OGG: 9,186 bytes in 0.1 seconds, verified by ffprobe.
Other ways into MKV, and what they measured
Among the published routes into MKV, 3GP is the eighth largest output of the 9 measured. The witness files differ, so this ranks the probe run and not your document.
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.
MPG to MKV: 34,577 bytes in 0.2 seconds.
What we will not pretend about 3GP to MKV
A 3GP file over 150 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A 3GP 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 3GP 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 3GP to MKV came out at 0.3 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 3GP running time and twice it, not the tenth of a second the probe file took.
A 3GP 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.
3GP to MKV is the single conversion here that is not charged at the floor rate. It costs 2 credits per minute of 3GP, every started minute counts, and because the engine refuses past 120 seconds one 3GP 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. 3GP 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 3GP. No transcription, no scene detection, no summary: 3GP to MKV is a container and codec operation, and the MKV says exactly what the 3GP said, in a different wrapper. The picture is the whole payload.
One 3GP file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
3GP to MKV: what people ask
What actually converts my 3GP 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 3GP file is uploaded once, ffmpeg runs once, the MKV comes back, and neither file is kept afterwards.
How long does 3GP to MKV take?
The probe measured 0.3 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 3GP running time and twice it. The 3GP is refused past 120 seconds, so the worst case is bounded.
What do I lose going from 3GP to MKV?
The one to know about first: Every pixel, one more time. The 3GP is decoded to raw frames and re-encoded as MKV, so whatever the 3GP 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 3GP?
Because the engine caps every video output at 1280 pixels wide. Encoder cost grows with pixel count, and a 4K 3GP would cost about sixteen times a 1080p one on a machine that also serves everybody else. A 3GP at or below 1280 pixels comes back at its own size, untouched.
Is 3GP to MKV free?
There is a free allowance every month, and 3GP to MKV is the one conversion here that is not charged at the floor rate: 2 credits per minute of 3GP, 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.