Upload a WebM 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.
WebM is a subset of Matroska, so everything in a WebM fits an MKV by definition. The tool copies the streams across with no quality change, almost instantly.
MKV opens doors WebM keeps shut: adding more audio tracks, subtitles, or chapters later, and smoother acceptance by media servers and archival tooling that treat MKV as the standard flexible container.