Compress an MKV file
MKV files are usually large because MKV is the container people reach for when a file is going to be large. That also makes them the case where a browser-based compressor has a real advantage: there is no upload, so there is no upload limit.
Drop your video here
or click to browse — MP4, MKV, MOV, WebM, AVI and more
- Typical codec inside
- H.264, HEVC or VP9; often multiple audio and subtitle tracks
- Output here
- H.264 in MP4 by default; MKV output also available
- Remux available
- Yes — MKV to MP4 in seconds if the video is H.264
Why MKV files get so big
Matroska is the container people choose when they want it to hold everything: several audio tracks, several subtitle tracks, chapter markers, attached fonts. Nothing about that flexibility makes video bigger by itself, but it correlates strongly with the things that do — MKV is where long recordings, disc rips, multi-hour streams and archival masters live.
OBS and other recording software often default to MKV rather than MP4 for a good reason: an MP4 that stops being written mid-recording, because the machine crashed or the power went, is frequently unrecoverable, while an MKV usually survives it. The consequence is that a lot of screen and stream recordings arrive as MKV files running to many gigabytes.
The upload limit problem, and why there is not one here
Free online compressors cap what you can send them — commonly at 100, 200 or 500 MB — because every uploaded byte costs them bandwidth and every encode costs them CPU. A four-gigabyte MKV is precisely the file those tools refuse.
This one has no such cap, because nothing is sent anywhere. FFmpeg is compiled to WebAssembly and runs inside your browser tab, so the only ceiling is how much your device can hold in memory. Large files are genuinely slower — a browser tab is not a render farm — but they are not rejected. If you have a 4 GB MKV and a machine with the RAM for it, it will work.
MKV to MP4 in seconds, without re-encoding
Browsers do not play arbitrary MKV files. Chrome and Safari will refuse most of them, which is why an MKV destined for a web page or a phone has to become something else first.
If the video inside is already H.264, that conversion costs nothing. Choose Remux with MP4 as the output: the video stream is copied bit for bit into a new wrapper and the job finishes in seconds on a file of any length, with zero quality loss. This is the correct answer far more often than people expect — a great many "I need to convert this MKV" problems are container problems, not codec problems.
What gets dropped when you convert
This is the honest caveat for MKV specifically. An MKV carrying five audio tracks and three subtitle tracks does not come out the other side with all of them. The conversion here takes one video stream and one audio stream — FFmpeg's default selection, which picks the highest-quality of each — and discards the rest. Subtitle tracks, chapters and attachments do not survive.
If those extra tracks matter, this is the wrong tool for the job and a desktop application such as MKVToolNix or HandBrake is the right one. If you are turning a recording into something you can send to a colleague, they almost certainly do not matter.
Settings for an MKV
If you only need it to play somewhere: Remux to MP4. If you need it smaller: Best compression to MP4, resolution set to whatever it will actually be watched at, quality slider in the middle to start.
For a multi-hour recording, be aggressive about resolution and gentle about bitrate. Thirty minutes of 480p lecture capture is watchable at a few hundred kbps because the frame barely changes; the same budget at 1080p is not. The 100 MB target page works through that case. If only part of the recording is worth keeping, set the start and end points first and encode that range alone.
MKV is also the one container here that accepts every codec on offer: H.264, H.265 and VP9. For an archive that lives on your own drive and never gets sent anywhere, H.265 in MKV is a defensible choice. Be ready for it to take several times as long as H.264, and check that whatever plays your library can actually decode it before you convert a shelf full of files.
MKV compression questions
Why will my browser not play an MKV?
Because browsers only ship support for a small set of container and codec combinations, and Matroska is not usually among them. The video inside may be perfectly playable — it is the wrapper that is not recognised. Remuxing to MP4 fixes it without touching the video.
Can I compress a 4GB MKV here?
If your device has the memory, yes. There is no upload and therefore no upload limit. Expect it to take a long time — you are running a video encoder inside a browser tab — and expect a phone to struggle where a desktop will not.
Will my subtitles survive the conversion?
No. Only one video stream and one audio stream are carried over; subtitle tracks, chapters and attachments are dropped. If you need them, use a desktop tool such as MKVToolNix or HandBrake instead.
Compressing to a size limit instead?
If what brought you here is a number rather than the fact that it is a MKV file, start from the ceiling instead. Each of these opens the compressor with that target already set, ready for your MKV:
Other formats
- Compress MP4 The universal container. Usually already H.264, so the gains come from bitrate and resolution.
- Compress MOV iPhone and QuickTime recordings. Frequently HEVC or ProRes, and frequently huge.
- Compress WebM Already efficient — so the realistic gains are smaller, and worth understanding first.
- Compress AVI Legacy codecs, dramatic savings. Read in, MP4 out.
- Compress WMV Old Windows exports. Nothing plays them now, and remuxing is not an option.
- Compress FLV A dead container, often with a perfectly modern video inside. Try remux first.
- Compress M4V Apple's MP4 under another name. Renaming it often solves the whole problem.
- Compress MTS / M2TS AVCHD camcorder footage. Enormous, usually interlaced, and split into 2 GB pieces.
The formats reference puts MKV beside the other containers and shows which codec combinations play where.