How to Convert MP4 to MKV

Before you start

Check where the conversion will happen. Some workflows use a locally installed video tool, while others upload the file to an online converter. The choice affects speed, file size limits, and privacy, so confirm the tool's local or upload-based operation before selecting a source file.

Make sure you have the original MP4 file available and enough free space for the converted MKV. A remux-style conversion can be faster because it copies the video and audio streams into a new container, but this depends on the tool and the chosen settings.

Step-by-step instructions

Convert locally

• Open the video converter or multimedia tool on your computer.
• Select the MP4 file you want to convert.
• Choose MKV as the output container.
• Review the video and audio settings before continuing.
• Start the conversion and wait for the process to finish.

If the tool offers a copy or remux mode, use it when you only need a container change and do not want to re-encode the media streams. This is usually faster and avoids an unnecessary quality loss. If the tool only shows codec or bitrate options, the conversion may re-encode the file.

Behavior can differ between tool versions. Some versions open the container choice, codec settings, and output location on separate screens, while others combine them into one dialog. Confirm the destination folder before starting so the finished MKV is easy to find.

Check the output

• Open the destination folder and confirm that a new MKV file was created.
• Play the first few seconds and a later section of the video.
• Check that audio plays and remains synchronized with the picture.
• Compare the duration shown by the player with the duration of the original MP4.

A container change may expose compatibility limits instead of changing the internal video quality. If the streams were copied, picture and audio quality should match the original. If the file was re-encoded, visible changes depend on the selected encoder and settings.

Common problems and next steps

If the player cannot open the MKV file, the issue may be the installed codec or the specific stream types inside the container. Try a media player with broad format support or inspect the file with a media information tool to see which video and audio formats are present.

If the conversion is slow, the tool may be re-encoding instead of copying the streams. Look for a copy, remux, or direct stream copy option. For a large file that must be converted remotely, check the service's stated size limit and connection requirements before uploading it again.

If the result has no audio or no video, the selected output may not support one of the original streams. In that case, choose a different codec for the missing track or use a tool that copies both streams into the MKV container.