MKV to MP4, copied in seconds

Remux: a new box, the same video

Remuxing means taking the encoded video and sound out of one container and writing them into another, packet by packet, without decoding anything. Quality cannot drop, because the pictures are never redrawn, and it is quick because nothing is computed. In our test, an eight-second 720p recording in MKV became an MP4 in a few dozen milliseconds, and every video and sound packet in the MP4 was identical to the MKV's.

Whether an MKV can be remuxed depends on what is inside:

Large MKV files: what we measured

MKV recordings get big fast: an hour at 6 Mbps is about 2.7 GB, and an evening of gameplay several times that. The tool reads the MKV from your disk a little at a time, so even a huge input is never loaded whole. We timed a 253 MB, ten-minute 720p MKV: it became an MP4 in about 0.6 seconds in both Chrome and Edge, with all 18,000 video packets and 28,126 sound packets carried across and a clean decode afterwards.

The output is the part to plan for. In Chrome and Edge on a computer, an MP4 over 500 MB goes straight into a file you pick, with no copy kept in memory, so the browser never has to hold a recording of several gigabytes at once. Other browsers, and phones, build the MP4 in memory before the download, which can fail for files of several gigabytes. The biggest file we have run is that 253 MB one; larger ones take the same path, so use Chrome or Edge on a computer for them, or cut the recording into parts first.

OBS recordings

OBS recommends recording to MKV, because an MKV is not lost as a whole when a recording does not stop cleanly, for example after a crash (OBS). The same guide shows OBS's own way to get an MP4: File, then Remux Recordings, or Automatically remux to mp4 in the Advanced settings. If OBS is open on the computer that made the recording, that is the quickest route.

Use this page when OBS is not at hand: on another computer, on a phone, or for a recording a friend or a client sent you. When the recording holds H.264 or HEVC video with AAC sound, the result here is the same kind of remux.

Subtitles, chapters and extra sound tracks

An MKV can hold many tracks: several languages of sound, subtitle tracks, chapters and attached fonts. The MP4 made here gets one video track and one sound track, the MKV's main ones, and leaves the rest out. The main sound track is the one the MKV marks as the default, or else the one with the highest bitrate. If you recorded game sound and your microphone on separate tracks in OBS, only that one comes across, so mix them into one track in OBS if you need both in the MP4.

Dolby Digital and other sound the browser cannot read

MKV files from a TV recorder or a disc often carry Dolby Digital (AC-3) or DTS sound. Turning that into AAC means decoding it first, and in our test Chrome on a Windows laptop could not decode AC-3. When that happens the tool says so before it starts and offers a way out: tick Leave the sound out, and the picture is still copied into a silent MP4. The video itself was untouched in that test, packet for packet.

Windows itself has the same gap now. Microsoft says that beginning with Windows 11, version 24H2, the AC-3 codec is no longer included with Windows, although many device makers pre-install one and PCs upgraded from an earlier version keep it (Microsoft).

What the MP4 leaves behind

Recorders write their name, their settings and sometimes a title into an MKV. None of it is written into the MP4. The new file holds the video and sound and the few details a player needs, and nothing that came from your device or the recording session.