Upload an MKV file and convert it to MP4. When the codecs already fit the target container, the conversion is lossless and near-instant; otherwise the video is re-encoded.
Your file never leaves your device. All processing happens right in your browser, and nothing is uploaded to a server.
Most MKV files carry H.264 or H.265 video with AAC or AC-3 audio, and MP4 accepts those directly. In that common case the tool copies the streams into the MP4 container with zero quality loss, finishing in seconds even for movie-length files.
When a stream genuinely does not fit MP4, it is re-encoded automatically at high quality. Either way you end up with a file that plays where MKV would not.
MKV is a great archive container with weak device support: TVs, phones, editors, and messaging apps regularly refuse it. MP4 is the universal currency of video. Converting changes the wrapper, not what your video looks like.