Turn an MKV into an MP4 that phones, editors, and upload forms accept. Most MKV files already hold H.264 or HEVC video, so the video is moved into the new container rather than compressed again.
Why MKV files will not open
MKV (Matroska) is a container that holds almost any codec, several audio tracks, and subtitles in one file. That flexibility is why many players, phones, and video editors do not accept it, while MP4 is supported nearly everywhere.
The picture inside an MKV is usually already in a format MP4 supports. Only the wrapper has to change.
Copied and re-encoded streams
- H.264 or HEVC video: copied unchanged.
- VP9 or AV1 video: re-encoded to H.264 at the source size.
- AAC or MP3 audio: copied unchanged.
- Opus, FLAC, or Vorbis audio: re-encoded to AAC.
The Plan section shows which applies to the loaded file before you convert.
Tracks that are left out
An MKV can carry several audio tracks and subtitle tracks. The MP4 gets the first video track and the first audio track only. Burn-in or separate subtitle files are not produced, and chapter markers are not copied.
Surround sound from film releases
AC-3, E-AC-3, and DTS soundtracks cannot be decoded by most browsers. When the first audio track uses one of them, the MP4 is written without sound and the page says so. Convert the audio separately in a desktop tool in that case.