ALL VIDEO TOOLS23
MP4 to MKV
Convert MP4 to MKV Online — Free & Lossless
Rewrap an MP4 as Matroska MKV without touching the video data.
About MP4 to MKV
Matroska is the container people reach for when MP4's rules get in the way: it takes almost any codec, any number of audio tracks, and subtitle and chapter structures MP4 handles badly or not at all. It is also what a lot of media-server and home-theatre software prefers. Going from MP4 to MKV is the easy direction — Matroska can hold whatever MP4 was holding — so the conversion is a pure container swap. The compressed video and audio are copied into the new wrapper byte for byte, nothing is decoded, and the run finishes in a few milliseconds no matter how long the clip is. Because the streams are untouched, the output is exactly as good as the input; the only thing that changes is the box around it, and the file size shifts slightly because the two containers keep their bookkeeping differently. The tool probes the file first and reports what it actually did, so the lossless claim is never made about a run that was not. Everything happens locally through FFmpeg compiled to WebAssembly.
Questions
Why convert to MKV at all?
Matroska takes almost any codec, any number of audio tracks, and subtitle and chapter structures MP4 handles badly or not at all. Media servers and home-theatre software generally prefer it. Going MP4 to MKV is the easy direction — the destination can hold whatever the source was holding.
Is anything lost?
No. This is a container swap: the compressed video and audio are copied into the Matroska wrapper untouched, nothing is decoded, and the output is exactly as good as the input.
Why did the file size change?
Only the wrapper changed. MP4 and Matroska store their indexes and headers differently, so the overhead around the video shifts by a few hundred bytes to a few kilobytes in either direction. The video data itself is identical.
Can MKV hold H.265?
Yes — that is rather the point of Matroska. Whatever codec your MP4 carries, including H.265, moves across as-is. Bear in mind that the file is no more or less playable than it was; only the container changed.
How big a file can I convert?
1.5 GB. The rewrapped copy is built in the browser tab, and that is the ceiling a tab can hold in a single file — so the limit is checked before the run rather than hit during it.
Is my file uploaded to a server?
No. Transmute processes everything locally in your browser using JavaScript and WebAssembly. Your files never leave your device — there is no server, no upload, no cloud processing.