Upload an MP4 file and convert it to MKV. 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.
Matroska accepts virtually every codec, so converting MP4 to MKV is a pure repackaging: video and audio are copied bit-for-bit into the new container. No quality change, no long processing.
MKV shines for archiving and flexibility: multiple audio tracks, soft subtitles, chapters, and robust handling of unusual codecs. Media-server users standardize on it for exactly those reasons.