Convert MP4 to MKV Free Online
Convert MP4 videos to Matroska MKV format for free in your browser. When the video is already H.264, GrepCut remuxes instead of re-encoding. No upload required.
Choose MKV when you want a flexible container for a local library rather than the safest sharing format. Matroska is commonly used for files that may later need subtitles, multiple tracks, chapters, or richer media metadata.
MP4 is still the easier choice for phones, browsers, social uploads, and built-in TV players. If your target device expects MP4 with H.264 video and AAC audio, converting to MKV may not improve playback.
The key detail is simple: MP4 and MKV are containers. They wrap video and audio streams, but they do not define the picture quality by themselves.
A remux changes the wrapper around the media streams. It does not recompress the video or audio, so it avoids the extra quality loss that can come from another encode.
When your MP4 contains H.264 video and AAC audio that GrepCut can copy, the output MKV keeps those compatible streams instead of rendering a new version of the picture.
If your MP4 uses a stream GrepCut cannot copy in this converter path, the result is no longer a pure remux. GrepCut converts the main playable track to H.264/AAC locally so you still get a usable MKV.
If your MP4 is a client recording, family clip, lecture, screen capture, or rough cut, uploading it just to change the container can be unnecessary risk.
GrepCut runs the conversion in your browser on your device. Your source file does not need to leave your computer for a simple MP4 to MKV job.
Local processing also helps with large files because you do not have to wait for an upload before conversion starts. Your device does the work, and you save the result from the same session.
Can you convert MP4 to MKV without losing quality?
Yes, when your MP4 contains compatible H.264 video and AAC audio that GrepCut can remux. In that case the streams are copied into MKV without re-encoding. If your source has to be transcoded, the output is re-compressed instead.
Will your MP4 be uploaded during conversion?
No. GrepCut processes your file in your browser, on your device. Your video does not need to be sent to a server to create the MKV.
Does converting MP4 to MKV make the file smaller?
Not usually when GrepCut remuxes. A remux mostly changes the container, so the file size should stay close to the original. If GrepCut transcodes, the size can change because the video and audio are compressed again.
Is MKV better than MP4 for Plex or Kodi?
MKV can be a better fit when your library depends on flexible track, subtitle, or archive workflows. MP4 can be better when your playback device prefers a simpler container. Test your target player if direct playback matters.
Can you keep subtitles when you convert MP4 to MKV?
MKV supports rich subtitle workflows, but this converter should not be treated as a full multi-track muxing tool. Use GrepCut for the playable video conversion, then use a dedicated subtitle workflow if you need advanced track packaging.
Why does your converted MKV still fail on a device?
The container is only one part of playback. Your device also has to support the video codec, audio codec, resolution, bitrate, and subtitle type. If the target app is strict, MP4 with H.264/AAC may still be the safer export.
Can you remux any MP4 to MKV in GrepCut?
No. GrepCut can remux when the source streams match the converter path, such as compatible H.264 video and AAC audio. If the source cannot be copied safely, GrepCut converts the main playable track locally instead.