Convert MKV to MP4 without re-encoding
basic conversion
// what it does
Stream-copies every selected stream (-c copy) from the Matroska file straight into an MP4 container without decoding or re-encoding. It runs in seconds and is bit-for-bit lossless because the video and audio packets are just rewrapped. Use it when an MKV holds H.264/H.265 + AAC and you only need MP4 for a player or device that won't open MKV.
// shell
$ ffmpeg -i input.mkv -c copy output.mp4// gotcha
The usual blocker is subtitles: MP4 has no tag for SubRip or ASS/SSA, so with this bare command they're silently dropped, and forcing them with -map 0 aborts the mux with 'Could not find tag for codec subrip ... not currently supported in container'. Convert subtitles to mov_text or leave them out. Modern MP4 does accept FLAC, PCM, and Opus audio, so audio is rarely the problem.