Convert WebM to MKV container or codec. Optimized for playback, editing, or web streaming.
Initializing WebAssembly engines
How do I convert WebM to MKV?
Drop your WebM file into the converter above. Make sure "MKV" is selected in the format dropdown. Click "Convert All" and then download your converted file.
Is it really free?
Yes — completely free. Convium is ad-supported. No account, no file limits.
Are my files private?
100% private. Files never leave your device. Everything runs locally in WebAssembly — we have no servers that touch your files.
File size limit?
No server-side limit. Your browser RAM is the only constraint — modern desktops handle multi-GB files. On mobile, keep it under ~500MB.
Why do I need a local server for video/audio?
Opening HTML via file:// blocks SharedArrayBuffer. Run: npx serve . or use VS Code Live Server.