Convert MKV to MP4

MP4 carries the video and the primary audio track. Extra audio tracks and embedded subtitles that MKV can hold are not kept — if you need a specific audio track, select it in a player first. For subtitles, convert them separately with the subtitle tools.

No. It is transcoded with WebCodecs on your own device — nothing is sent to a server, so there is no account, no quota and no size limit. Very large 4K files depend on your device’s memory. Chrome, Edge and Firefox work best.

It is a re-encode to H.264, so technically a lossy step, but at the tool quality setting a single pass is hard to tell from the source. The point of the conversion is not to improve the picture but to move the same video out of a container many devices refuse and into MP4, which plays almost everywhere. Start from the best MKV you have rather than an already-converted copy.

MKV (Matroska) is a flexible, high-quality container popular for downloads and archiving, but it was never widely adopted by phones, browsers and social platforms, which standardised on MP4. The video inside is often perfectly ordinary — converting to MP4 keeps that same picture and just wraps it in the container those devices and sites expect.

It still re-encodes rather than simply repackaging, so it is not instant — the tool decodes the video and encodes a fresh H.264 MP4. That is what guarantees a clean, standards-compliant file that plays everywhere, even when the source used an unusual profile or settings. Expect it to take roughly as long as the clip length and size warrant, since the work happens on your own machine.