Compress MKV
Reads the MKV on your device and writes a smaller MP4 or WebM.
When you load an MKV here, the After size is filled in with a size computed from the video's resolution and frame rate. You can tap 70%, 50%, 30% or 10% instead, or click the After size and type an exact number in MB.
Free Video Compressor
No sign-up. No upload. No size limit.
Nothing is uploaded — the file is compressed on your device.
What is inside an MKV
MKV (Matroska) is a container. It holds a video stream, usually H.264 or HEVC, plus one or more audio tracks, and sometimes subtitles and chapters. The size comes almost entirely from the video bitrate, so an MKV shrinks the same way any video does: the video is decoded and encoded again at a lower bitrate.
There is no MKV output. The result is MP4 or WebM and contains the video and the first audio track. Subtitle tracks, chapters and any second audio track are left out, so keep the original if you need them. The original file is never changed; the result is a new file.
OBS recordings
OBS describes MKV as failsafe: a recording cut off by a crash or power loss stays playable (OBS formats guide, checked September 2026). Since OBS 32.0, new profiles default to Hybrid MP4 (Hybrid MOV on macOS), so an MKV from OBS usually comes from an older profile or a format you picked yourself.
OBS can remux an MKV to MP4. Remuxing copies the same streams into a new container, so the file stays about the same size. To make it smaller the video has to be re-encoded, which is what this page does.
If your recording has separate mic and game audio tracks, only the first one is kept. Mix them in OBS or an editor first. A 60 fps capture can go to 30 fps under Advanced → Frame rate. For a long session where you need one part, trim it ↗ first.
HEVC, surround audio and large files
HEVC inside an MKV can only be read on a device with a hardware HEVC decoder, because Chrome has no software HEVC decoder. Audio with more than two channels (5.1 and up) is mixed down to stereo when it is re-encoded. The output is written to disk while it is produced, so an MKV of several GB works, and nothing is uploaded.
Frequently asked questions
Can the output be MKV too?
No. Advanced → Format offers MP4 (H.264), MP4 · HEVC, WebM (VP9) and WebM · AV1, and only lists the ones this browser can encode on your device.
I remuxed my OBS recording to MP4. Why is it just as big?
Remuxing moves the streams into a new container without touching the bitrate.
Are subtitles and chapters kept?
No. Only the video and the first audio track go into the new file.
My MKV will not load. Why?
If the video inside is HEVC, the device needs a hardware HEVC decoder to read it; Chrome has no software one. Try the file on a device that has one. AVI, WMV and FLV files are not supported and show "This file format is not supported."
Compress MKV
Opens the compressor with a target size computed for this page.