Convert MP4 files to the Matroska MKV container, the format of choice for media servers, Plex libraries and anything that needs to carry several audio tracks at once. Usually lossless and near-instant, because your existing streams are repackaged rather than re-encoded. Everything runs instantly in your browser: nothing is uploaded and no account is needed.
or click to select a file
🔒 Your files never leave your device - 100% private & secure
Drag an MP4 onto the upload area or click Select Video. Large library files are fine, because nothing is sent to a server.
MKV is selected automatically. Click Convert - most files repackage without re-encoding, so it finishes in seconds. Download the MKV when done.
Matroska is the most permissive of the common containers. Where MP4 is fussy about what it will carry, MKV will hold practically any combination of video, audio and subtitle streams in one file, which is why media servers and home libraries standardise on it. If you are building a Plex, Jellyfin or Emby library, MKV is usually the format that causes the fewest surprises.
The practical draw is multiple tracks. A single MKV can carry the original audio alongside a commentary or a second language, plus several subtitle tracks, and let the player choose between them at playback time. MP4 can technically do some of this, but support across players is patchy enough that many tools simply ignore the extra tracks.
Because MKV accepts essentially every codec, converting into it almost never requires touching the video. The H.264, HEVC, AV1 or VP9 stream inside your MP4 is written into the Matroska wrapper exactly as it is, along with the audio, so the conversion is lossless and finishes about as fast as your disk can write the file.
The one trade-off worth knowing is compatibility in the other direction: MKV is superbly supported by desktop players like VLC and by media servers, but far less so by phones, smart TVs and browsers. If you need a file that plays anywhere without thinking about it, MP4 remains the safer container. Everything here runs locally in your browser, so whichever way you go, your files stay on your device.
No. Matroska can carry the same codecs MP4 uses, so the existing video and audio streams are copied into the new container without being decoded and re-encoded. The result is bit-for-bit identical to the source.
Because it holds multiple audio and subtitle tracks reliably and accepts any codec, so a server can store one file per title rather than several variants. Plex, Jellyfin and Emby all handle MKV as a first-class format.
Often, but not always. Desktop players like VLC handle MKV everywhere, while phones, smart TVs and browsers vary. If broad playback matters more than multiple tracks, convert to MP4 instead.
There is no hard limit. Because nothing is uploaded, the only practical ceiling is your device's available memory. Most computers handle multi-gigabyte files without issue.