What is this MP4 to MKV converter?
A free, browser-based tool that converts a MP4 video to MKV. Add a file, preview it inline, and download the result. All processing happens locally in your browser, so your video is never uploaded and there is no watermark.
Folding MP4s into an MKV-first media archive
If you maintain a media library (a Plex or Jellyfin server, a course catalog, a shelf of finished projects) mixed extensions become a tax on every later operation. Standardizing on MKV is a common endgame because the container is a superset of what MP4 offers: whatever you decide to attach in the future (an SRT or ASS subtitle track, a commentary or second-language audio track, chapter markers, even font attachments) MKV has a defined slot for it, while MP4 supports only a narrow subset of that list. Converting existing MP4s is the cheap part of the migration, which is the point of this pair.
MP4 to MKV moves streams, not pixels
Everything an MP4 can legally contain (H.264, H.265, AV1, and their usual audio companions) is also legal in MKV, so this conversion is a remux in every realistic case. The streams transfer byte for byte, the operation completes in about the time of a file copy, and quality is mathematically unchanged. File size stays essentially where it was as well, since only the thin container layer around the data is rewritten. That makes the conversion safe to run across a whole library in one sitting: there is no lossy step to regret later.
How to use it
- Add a MP4 file on the left (drag & drop or click) — it shows an inline preview.
- Pick a quality preset (used only when the video has to be re-encoded).
- Press “Convert & download” to get your MKV file. The first run downloads the ~30MB converter once.
Lossless remux vs re-encoding
MP4 and MKV are containers — what matters is the video codec inside. When the codec already fits MKV, this tool just repackages it (a "remux"): instant and completely lossless. Only when the codec isn't compatible — most often anything involving WebM — does it re-encode the video locally, which takes longer and is where the quality preset applies.