Formats & playback 5 min read
MKV to MP4: When Remuxing Is Lossless and When You Must Re-encode
MKV and MP4 are containers — wrappers around the actual video and audio streams. Whether conversion is lossless depends entirely on which codecs are inside the MKV, not on the MKV label itself.
Remux (lossless, seconds-fast) when
- Video is H.264/AVC or H.265/HEVC — both are legal inside MP4.
- Audio is AAC or MP3 — also MP4-legal.
- Subtitle tracks are limited to mov_text; anything fancier gets dropped or burned separately.
Re-encode when
- Audio is DTS, TrueHD, FLAC or AC3 — MP4 players commonly reject them; convert to AAC.
- Video is VP9/AV1 in an MKV — technically possible in MP4 but widely unsupported by hardware players; H.264 is the safe target.
- Attachments, chapters or multi-angle tracks exist — MP4 has no equivalent, so they are lost in remux.