How to Convert MKV to MP4 (and What You Might Lose)
MKV (Matroska) is a flexible container popular for archived and downloaded video. It can hold many audio tracks, subtitle tracks and chapters in one file. That flexibility is also why MKV files are not accepted everywhere: a number of TVs, phones, editing programs and websites do not support them, or support only some kinds of MKV files.
Convert in your browser
- Open the converter and add your MKV file.
- Keep MP4 selected in "Convert to".
- If the MKV is very large or high resolution (for example 4K), consider choosing 1080p to keep conversion time and memory use reasonable.
- Click Convert, then download the MP4.
What gets kept and what does not
Be careful here, because MKV files often contain more than a single picture and a single soundtrack.
- Video: Re-encoded as H.264 in the MP4 file.
- Audio: One audio track is kept and re-encoded as AAC at 128 kbps. If your file has several languages or commentary tracks, you cannot pick which one is used on this site, and the others are not guaranteed to be kept.
- Subtitles and chapters: Do not rely on them being kept. If subtitles matter, keep the original MKV, or use a desktop tool that lets you select tracks.
- HDR or 10-bit video: The output is standard 8-bit video (
yuv420p). Colors in HDR footage may look washed out when it is converted without proper tone mapping, which this site does not do.
Re-encoding vs remuxing
Because MKV and MP4 are both containers, a file whose video is already H.264 and whose audio is already AAC can often be moved from one container to the other without re-encoding. This is called remuxing. It is very fast and loses no quality. A desktop FFmpeg command for this looks like:
ffmpeg -i input.mkv -c copy output.mp4
Remuxing only works when the codecs inside are allowed in MP4. Free Video Converter does not offer it; it always re-encodes, which takes longer and can slightly reduce quality, but it will produce a standard file even if the original codecs are unusual. If your priority is to keep every track and the exact original quality, use a remux with a desktop tool. If your priority is to get a file that plays on a device that rejects MKV, a quick re-encode in the browser is simple and good enough.
Common problems
- "Conversion failed": Try a lower resolution or close other browser tabs. Very large files can exhaust the memory available to the page. See troubleshooting.
- Result has no subtitles: See above; use another tool if you need them.
- Audio is in the wrong language: The wrong track was chosen by default. Use a desktop tool to select the track you want first.
Is MKV bad?
Not at all. For storing a film with several languages and subtitles, MKV is arguably the best container. Convert to MP4 when you need compatibility, not because MKV is inferior. For help deciding, see the format comparison.
Ready to try it? Convert your video in the browser, no upload and no sign-up.
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