MKV is a fine container but many phones, smart TVs and editing programs refuse it. Converting to MP4 keeps the video and makes it play everywhere.
Where the video and audio inside are already MP4-compatible the streams are copied rather than re-encoded, which is both instant and lossless. Otherwise the file is re-encoded to H.264 and AAC.
Everything happens on your device, so a personal recording is never uploaded to a stranger’s server just to change its container.
How to use it
Add your MKV file.
Press Convert — where the video inside is already H.264, the streams are copied rather than re-encoded, which is fast and lossless.
Download an MP4 that plays on phones, TVs and editors.
Supported formats and limits
Input
MKV (Matroska)
Output
MP4
Speed
Very fast when streams can be copied instead of re-encoded
Quality
Unchanged in the copy case
Subtitles
Embedded text subtitles may not survive the container change
Where it runs
In your browser; nothing uploaded
Not for: Very large films on a phone — browser memory limits the size; a desktop handles those far better.
Frequently asked questions
Are my files uploaded to a server?
No. This conversion runs entirely inside your browser using your own device’s processing power. The file never leaves your computer or phone, which is why it also works when your connection drops.
Will I lose quality?
Not when the streams can be copied. If a re-encode is needed there is some loss, as with any re-encode, but at default settings it is hard to see.
What about subtitles?
Subtitle tracks embedded in an MKV are not carried into the MP4. Extract them separately if you need them.