How to convert mkv to mp4 without losing quality in windows 11?

You can't convert mkv to mp4 with Windows Media Player as it is is designed primarily as a media player, not as a transcoding or container-conversion tool. Here's why you can’t convert MKV to MP4 with it:

No Built-In Conversion Engine

WMP doesn’t include any feature to repackage or re-encode video files. It can play formats it has codecs for, but it won’t let you export or save them in a new container or with new codecs.

Limited Container Support

Out of the box, Windows Media Player doesn’t support the MKV (Matroska) container at all—so it can’t even open your source file to begin with. MP4 support is similarly restricted to certain codecs (like H.264 video and AAC audio) via installed Windows codecs.

No Codec Management

Even if WMP could open MKVs via extra codec packs, it still lacks any way to switch video or audio codecs and repack into MP4. Conversion requires a tool that can decode the source streams and then encode them into your desired formats—and WMP simply doesn’t do decoding/encoding workflows.

What to Use Instead

To convert MKV to MP4 on Windows 11, choose a dedicated mkv to mp4 converter all of which can open MKV files, decode the streams, and then either remux or re-encode them into an MP4 container.