Does my video get uploaded anywhere?

No. The video is decoded and re-encoded entirely in your browser. It never leaves your device, and once the page has loaded it works offline.

Why did my file barely shrink, or get bigger?

MKV is a container, not a compression setting. If the video inside was already encoded at a low bitrate, there is little left to remove, so re-encoding can land near the original size or slightly above it. Files with a lot of detail and a high bitrate shrink the most.

What is the difference between the three levels?

Smaller file uses the lowest quality target for the strongest compression, balanced is a sensible middle ground, and higher quality keeps more detail at the cost of a larger file. All three re-encode to H.264.

Will the quality drop?

Re-encoding is lossy, so some detail is always discarded — more at the smaller-file setting, less at higher quality. For the best result, compress from the original file rather than a copy that has already been re-encoded.

Why does it sometimes fail to compress?

Compression needs your browser to be able to encode H.264 video. Some browsers can't, in which case you'll see an error instead of a result. Try a recent version of Chrome or Edge.

Compress MKV

Re-encodes an MKV to H.264 at a lower quality target to shrink it, keeping the .mkv container.

What this does

This decodes your MKV and re-encodes the video stream to H.264 at the quality target you pick, then writes the result back into a Matroska (.mkv) container. The audio is re-encoded to AAC. A lower quality target spends fewer bits per frame, which is what makes the file smaller.

How much it shrinks depends on the source. A file that is already heavily compressed has little left to remove, so re-encoding it can keep it about the same size or even grow it slightly. Footage with a high bitrate has the most to gain. Re-encoding is lossy: each pass discards detail, so compress from the original whenever you can.

How it works

1Drop your MKV (or any video your browser can open) or pick it from your device.
2Choose a compression level: smaller file, balanced, or higher quality.
3Click Compress, check the before/after size, then download the MKV.

Built with open source

Mediabunny — Converts and edits video and audio in the browser via WebCodecs. Add-on encoders cover MP3, AAC, and FLAC. · MPL-2.0

Frequently asked questions

Related tools

All Compress →

Video CompressorCompress a video to a smaller file in your browser. Re-encodes to H.264 at the quality level you pick.Image CompressorCompress JPG, PNG, and WebP images. Each one keeps its format. Defaults aim for the smallest file you can't tell apart from the original.PNG CompressorCompress PNG images. Color quantization cuts real size, and there's a lossless option when you need it.JPEG CompressorCompress JPG and JPEG images in bulk. A quality slider lets you trade size for sharpness.Compress MOVCompress a MOV (QuickTime) video to a smaller file in your browser. Re-encodes to H.264 at the quality level you pick.