100% on-device
Compress an MKV in your browser
Make a heavy .mkv small enough to send or to keep — re-encoded on your own device, and handed back as an MP4 that plays anywhere.
How it works
- Drop your .mkv. Matroska is read directly; nothing is uploaded.
- Name a size. Type a limit and the tool solves the bitrate that lands under it, from your budget and the video's duration.
- Download an MP4. H.264/AAC — the format phones, browsers and chat apps actually accept.
MP4 cannot hold what Matroska routinely carries, and that is the real cost
An .mkv is built to carry an arbitrary number of tracks: several audio languages, a commentary, a stack of subtitles, chapter markers, attached fonts for styled ASS subtitles, and cover art. What comes back has one video track and one audio track. Styled subtitles with their embedded fonts have no MP4 equivalent at all, and the chapter marks go with them.
Remuxing rather than re-encoding would keep more of that, but it would not make the file any smaller - the video track is the size, and only re-encoding touches it. So keep the .mkv as the archive and treat the MP4 as a disposable copy for sending. Extract any subtitles you need with a Matroska tool before you start.
A disc remux and an x265 web encode shrink for opposite reasons
Matroska accepted H.265 and AV1 long before MP4 tooling did, so a modern .mkv often holds a codec already more efficient than H.264. Re-encoding that to H.264 at matched quality needs more bits, not fewer; it only gets smaller because you named a smaller size and accepted the quality that buys.
A disc remux is the opposite case. It keeps the original video bitrate untouched and carries lossless audio - TrueHD or DTS-HD MA, several megabits per second on their own - which is dropped here regardless. Knowing which of the two you have sets the expectation before you spend the encoding time, and the two really do behave nothing alike.
Questions
Is my video really not uploaded?
Yes — and not as a policy, as a fact of how it is built. There is no upload endpoint: the file is read straight into this browser tab and compressed by code running on your own machine. You can watch the Network tab while it works, or pull your Wi-Fi out mid-compression and it will still finish. Nothing is stored, nothing is queued, and there is no account.
What happens to my subtitles and extra audio tracks?
They are dropped, and you should know that before you start. An .mkv often carries several audio tracks and a stack of subtitles; the MP4 you get back keeps one video track and one audio track — the default ones — and nothing else. If those subtitles matter, keep the original .mkv. This tool is for making a video small and playable, not for preserving a Matroska archive.
Why not give me back an MKV?
Because an MKV is usually the reason someone is here: it will not open on a phone, will not upload to a chat app, and will not import into most editors. Handing you a smaller file with the same problem would not be a fix. H.264 in an MP4 plays everywhere.
A long film will take a while, won't it?
Yes — and honestly so. This runs on your machine, in your browser, on one CPU thread; a feature-length video is real work and will take minutes, not seconds. It keeps going as long as you leave the tab open, and the progress bar does not lie to you about how far along it is.
Other formats
The same workbench handles all of them — pick a tab above, or start from its own page: