Convert MKV to WebM
Turn MKV files, such as OBS recordings, into WebM on this device. Matching VP9 video and Opus sound are copied without re-encoding.
- Nothing is uploaded
- Works offline after first visit
- Open source
- No watermark · limit is your device’s memory; phones hold less
Converts MKV videos to WebM. It is free, needs no account and adds no watermark, and your video is not uploaded: your browser reads the file and writes the WebM inside this tab. Video already in VP8, VP9 or AV1 and sound already in Opus or Vorbis are copied as they are; anything else is re-encoded to VP9 (VP8 where the browser cannot write VP9) and Opus. The size limit is your device’s memory, and after one visit and a short wait the page works offline.
Done
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Open the network tab. Your files never appear in it.
Add files and run the tool to see every request this tab makes.
Your files are processed on this device and never sent to a server. Reload the page to clear everything from memory.
MKV (Matroska) and WebM are close relatives: WebM is a restricted form of Matroska that browsers play. That makes an MKV with VP9 video and Opus sound, which some screen recorders and OBS setups write, a file that can move into a WebM without being re-encoded. A WebM is the form a web page, a game engine or a chat app usually wants.
When the video and sound are already in codecs WebM allows, this page copies them across, so the result takes about as long as reading the file and the picture is untouched. Most MKVs are not like that: an OBS recording is usually H.264 with AAC sound, which a WebM cannot hold. Your browser then re-encodes the picture to VP9 (or VP8) and the sound to Opus with its own encoders. That takes longer than a copy, and like any lossy re-encode it can cost a little detail.
If the picture is HEVC and your browser cannot decode HEVC, or the sound is in a format it cannot decode, such as AC-3 or DTS, the page says so and writes nothing; tick Remove the sound to convert the picture alone. Only the main video and one sound track are kept, so extra sound tracks and subtitles are left out.
For a file that phones, TVs and editors open more widely, use MKV to MP4 instead; to make a large video smaller, use Compress video.
How it works
- Drop one or more MKV files, or tap to pick them.
- Tick Remove the sound if you want a silent WebM.
- Press Convert to WebM. One file downloads straight away; several download as a zip.
In your browser, not on a server
Mediabunny (MPL-2.0), a media toolkit written in TypeScript, reads the MP4, MOV, MKV or WebM container in your tab and writes a WebM. VP8, VP9 or AV1 video and Opus or Vorbis sound already in the file are copied across as they are. Anything else is decoded and re-encoded by your browser’s own encoders (WebCodecs), VP9 where the browser can write it and VP8 otherwise, with the sound as Opus. A browser with no VP8 or VP9 encoder says so instead of writing a broken file.
Compared with upload-based tools: The video is read from your disk and the WebM is written in memory on your device, so there is no upload to wait for and the only limit is how much memory your device has.
Verify it yourself. Give the page a short while after one visit to save itself for offline use, then turn off Wi-Fi or switch to airplane mode and run the tool again. Or open your browser's developer tools (F12, or Cmd-Option-I on a Mac), pick the Network tab, and watch it while you convert. The panel above does the same count for you.
Questions
How do I convert MKV to WebM?
Drop the .mkv file on this page and press Convert to WebM. Your browser reads the file and writes a WebM with the same name and a .webm ending. Nothing needs installing, and you can drop several files at once.
Is the video re-encoded?
Only when it has to be. VP8, VP9 and AV1 video is copied, and so is Opus or Vorbis sound. H.264 and HEVC video and AAC, MP3 or AC-3 sound cannot go into a WebM, so they are re-encoded by your browser. The line under the result says what happened.
Can I convert an OBS recording to WebM?
Yes. An OBS MKV that holds H.264 and AAC is re-encoded to VP9 and Opus, which takes longer than a copy. If you only need an MP4, MKV to MP4 copies those streams instead, with no re-encode.
Why does my browser say it cannot write WebM?
WebM needs a VP8 or VP9 encoder and an Opus encoder in the browser, and not every browser or Linux build has them. Chrome and Edge on a computer write WebM, and so does Safari’s engine on a recent Mac. The page says so rather than writing a broken file.
Is the MKV uploaded?
No. Your browser reads the file from your disk and writes the WebM with code running in this tab. Open the network tab, run a conversion, and your video is not among the requests.
Is the file uploaded anywhere?
No. There is no server in this process. The page is a static file and the work is done by code running inside your browser tab. Give the page a short while after one visit to save itself for offline use; then you can turn off Wi-Fi and it still works.
More video and audio tools
Video to WebM
Turn MP4, MOV and MKV files into WebM with VP9 video and Opus sound. Converted on this device, not uploaded.
Audio splitter
Cut one long recording into separate files at the times you choose or at equal lengths. Done on this device, and the file is not uploaded.
Video to MP3
Pull the sound out of a video, or turn any audio file into an MP3. Your files are converted on this device and never uploaded.
Video to GIF
Trim a clip, pick a size and frame rate, and get a looping GIF. The video is decoded by your browser and never leaves this device.