mkv → mp4
MKV to MP4, and the picture never gets touched
For the television, the phone or the editor that refuses to acknowledge MKV exists. Most files are done in seconds, because nothing about the video actually has to change.
Processed on your own hardware · never transmitted, never stored
What happens when you drop a file in
The file stays put. Your browser opens it the same way an image viewer would. There is no transfer, which is why there is no upload bar to watch.
An encoder arrives once. The first job on this site pulls down a compiled build of FFmpeg, roughly 30 MB. It is kept afterwards, so every job after the first starts immediately.
Your own hardware does the job. The work happens in this tab, on your processor. A laptop from the last few years handles short clips in well under real time.
The result lands in Downloads. Assembled in memory and handed straight to your browser. Close the tab and every trace of it is gone.
The container is not the codec
This confuses almost everyone at some point. MKV says nothing about how the video was compressed — it describes how the pieces are packed together. Open one up and you will usually find H.264 or H.265 inside, both of which MP4 is perfectly happy to carry.
So the job is rarely a conversion. The streams get moved from one wrapper into another, which takes seconds and leaves the image mathematically identical to what you started with.
MKV became popular precisely because it carries things MP4 handles badly: several audio languages, embedded subtitles, chapter markers. Those extras are what the move costs you — not picture quality.
What gets left behind
Subtitles baked into the MKV as a separate track do not survive, and only the first audio track comes across. A file holding both an English and a Japanese dub will give you whichever came first in the file. That is a limitation of MP4 itself rather than anything this page could work around.
Common questions
Does the picture get worse?
In the normal case, no. The video stream is copied without ever being decoded, so it is identical.
My subtitles vanished.
MP4 does not carry them the same way. Pull the subtitle file out separately before converting if you need it.
Why did mine take a while?
Some MKV files hold VP9 video or FLAC audio, neither of which MP4 accepts. Those get re-encoded automatically.
Is there a size limit?
Nothing enforced, but a browser tab runs out of memory somewhere between 500 MB and a gigabyte depending on the machine.