Compress MKV files and keep all their audio tracks
The MKV compressor re-encodes the video inside a Matroska (.mkv) file to make it smaller, and carries every audio track across — the original language, the dubs and the commentary — with their language tags. Choose a target size or quality and one of four codecs: H.264, HEVC, VP9 or AV1. It runs in your browser, so recordings, family videos and other personal files are never uploaded.
MKV is a flexible container: one file can hold several audio and subtitle tracks, fonts for styled subtitles, chapters and more. The browser engine this tool uses (Mediabunny, MPL-2.0, on WebCodecs) reads the video and audio tracks but not subtitles, attachments or chapters. Instead of losing them quietly, the tool scans the file’s track list and tells you exactly what will not be copied, before you press a button.
How to use the MKV Compressor
- Drop the .mkv file on the box or click to choose it.
- Read the details and the plan: the video codec, each audio track with its language, and any subtitle tracks, attachments or chapters that will be left out.
- Need the subtitles? Save them first with Extract Subtitles.
- Choose Target file size or Quality, a Video codec and what to do with the Audio: keep the tracks as they are or re-encode them at a lower bitrate.
- Press Compress video, check the result side by side and press Download MKV.
MKV Compressor features
Every audio track kept
All audio tracks go into the new file with their language codes, copied losslessly by default or re-encoded at the bitrate you choose.
Copy what browsers can’t decode
An AC-3 or DTS track your browser can’t decode is still copied as it is instead of being thrown away.
Four video codecs
H.264 for compatibility, HEVC or AV1 for the smallest files, VP9 as a royalty-free middle ground — each checked against your browser first.
A clear list of what’s dropped
Subtitle tracks (with their format), font attachments and chapters are read from the MKV header and listed before compressing.
Size target with retry
In target mode the bitrate is corrected and the video encoded again if the first pass is too big.
What is kept and what is dropped
After compressing, you can put text subtitles back with Add Subtitles to Video, or keep them as a separate .srt file next to the video — players such as VLC load a subtitle file with the same name automatically.
| Part of the MKV | What happens | Why |
|---|---|---|
| Main video track | Re-encoded with the codec you choose | This is where the space is saved |
| Audio tracks | All kept, with language tags | Copied unchanged by default |
| Subtitle tracks (SRT, ASS, PGS…) | Not copied — listed first | The browser engine doesn’t read subtitle tracks yet |
| Attachments (fonts) | Not copied — listed first | Only needed by styled subtitles |
| Chapters | Not copied — listed first | Not read by the browser engine |
| Extra video tracks | Not copied | Only the main video track is compressed |
Choosing a codec for MKV
Matroska accepts practically any codec, so the choice is about where you will play the file. H.264 works on virtually every TV, media box and player. HEVC needs roughly 40% less bitrate for similar quality and plays on most media players and TVs from recent years. AV1 is usually the smallest of all but encodes slowly and needs newer hardware to play smoothly. VP9 sits in between and plays in browsers and on Android.
Many MKV files — anime releases in particular — use 10-bit HEVC to avoid banding in smooth gradients. The browser encoders used here write 8-bit video, so very smooth skies or dark scenes can show slight banding at low bitrates; give such videos a little more bitrate.
Audio: keep or re-encode?
- Keep original copies every audio track bit for bit. It is lossless and instant, and the right choice when the audio is already compact (AAC or Opus).
- Re-encode pays off when a film carries large multichannel tracks. Surround audio is mixed down to stereo when it is re-encoded.
- In target-size mode the audio comes out of the same budget as the video, so several full-quality audio tracks leave less room for the picture.
- To pull a single soundtrack out of the file instead, use Video to Audio.
MKV Compressor FAQ
How do I reduce the size of an MKV file?
Drop it on the MKV compressor, choose a target size or quality and press Compress video. Lowering the resolution, for example from 1080p to 720p, saves the most.
Will I lose the other languages in my MKV?
No. Every audio track is kept with its language tag. Only subtitle tracks, font attachments and chapters are left out, and the tool lists them before you start.
How do I keep the subtitles?
Extract them first with Extract Subtitles, compress the video, then add them back or keep the .srt file next to the video.
Why does my AC-3 or DTS audio stay the same size?
Many browsers can’t decode these formats, so they can’t be re-encoded; the track is copied as it is so you don’t lose it.
Can I convert MKV to MP4 while compressing?
Yes — open the file in the video compressor and choose MP4 as the output format.
What if my MKV uses Xvid, MPEG-2 or another old codec?
Browsers can’t decode those, so the page offers to decode the video with ffmpeg.wasm (GPL-2.0-or-later, a one-time 32 MB download you confirm) and then compresses it with your browser’s encoders. Subtitles, fonts and chapters are not carried over on this path either.
Is my MKV uploaded anywhere?
No. The file is read and encoded by your browser on your own device. Only the video engine is downloaded from a CDN.