Reduce MKV file size with CRF re-encoding.
All processing happens in your browser — files never upload anywhere.
Compress MKV is a browser-based video utility powered by FFmpeg WebAssembly — the same production-grade transcoder that powers VLC, HandBrake, and countless streaming platforms, compiled to run on your device. First visit downloads about 30 MB of the FFmpeg core (cached after), then every subsequent use is instant and works offline.
FFmpeg WebAssembly
Every step runs on your device — nothing is uploaded.
Open Compress MKV. First-time visitors will see a one-time FFmpeg WASM download (~30 MB, cached forever).
Drop your video file. It stays on your device — no upload.
Adjust any settings shown (target format, quality, trim range).
Encoding takes roughly 30-60 seconds per minute of source video. Progress is shown.
Click Download to save the finished file.
Yes. Compress MKV loads FFmpeg WebAssembly (~30 MB, cached after first use) and does all encoding in your browser's memory. Nothing is uploaded to any server — you can even use it offline after the initial load.
Roughly 30-60 seconds per minute of source video on a modern laptop. Long videos (over 30 minutes) may run out of browser memory — for those, Toolspace's Trim tool can split first.
The full FFmpeg codec matrix: H.264, H.265, VP8, VP9, AV1, ProRes, and every audio codec you'll encounter. If your player accepts it, FFmpeg reads it.
Compress MKV uses FFmpeg's default encoding parameters (CRF 23 for H.264) — visually indistinguishable from the source in normal viewing. For master-quality output, drop the CRF or choose a lossless codec target.
No — because there's no server-side state, closing the tab cancels the run. On the plus side, restarting is as simple as dropping the file back in.
Native FFmpeg uses your CPU's SIMD instructions and (on many machines) hardware acceleration. WebAssembly FFmpeg loses about 2× to native but gains you complete privacy, zero installation, and no upload wait. For most short clips, that's a great trade.
All free, all browser-based, all zero-signup.