MKV (Matroska) is a flexible container, but many phones, TVs, video editors and websites only accept MP4. Most MKV files hold H.264 or H.265 video, and those streams can go into an MP4 unchanged: the conversion is a remux that moves the streams into the new file without touching the picture.
How to convert MKV to MP4
- Upload the MKV file (up to 2 GB).
- Keep Convert to: MP4 and Conversion: Automatic.
- Download the MP4. The result says what was copied and what was re-encoded.
What happens to each part of the file
- Video: H.264 and H.265 (HEVC) are copied unchanged, so the quality is identical and the conversion is fast. Other codecs (VP9, AV1, MPEG-2, MPEG-4 Part 2…) are re-encoded to H.264.
- Sound: AAC and MP3 tracks are copied. AC-3, E-AC-3, DTS, Opus, Vorbis, FLAC and other sound is re-encoded to AAC, which every MP4 player understands. Every sound track is kept, with its language.
- Subtitles: text subtitles (SRT, ASS, WebVTT) become MP4 text subtitles. Picture-based subtitles from Blu-ray or DVD rips (PGS, VobSub) can't be stored in MP4 and are left out; the result warns you.
- Chapters are kept.
Good to know
- An MP4 with H.265 video plays on newer phones, computers and TVs, but not everywhere. If an older device refuses it, convert again with Always re-encode: the video then becomes H.264.
- Copying works for videos up to 3 hours. When the video has to be re-encoded, the length is limited by processing time (about 25 minutes of 1080p); a lower maximum resolution under “More options” allows more.
- The file's own tags, such as the title, aren't copied into the MP4.
More about what this tool reads and writes: Video Converter.