MP4 to MKV

Repackage an MP4 file into an MKV container without re-encoding the video or audio streams. Because nothing is decoded — the existing streams are just copied into a new container — the conversion is very fast and there is no quality loss. MKV is a flexible open container that supports multiple audio tracks, subtitles, and chapters; it's popular for media libraries and home theater setups. The conversion runs locally in your browser via ffmpeg.wasm, so the file never leaves your device. For best results keep inputs under about 500 MB.