Are my files uploaded to a server?

No. This conversion runs inside your browser using your own device’s processing power, and the file is not uploaded. Once a tool has been used once (video, text recognition and background removal download their engine from this site the first time), it keeps working when your connection drops.

Not when the streams can be copied. If a re-encode is needed there is some loss, as with any re-encode, but at default settings it is hard to see.

What about subtitles?

Subtitle tracks embedded in an MKV are not carried into the MP4.

Convert MKV to MP4

Runs on your device · Your file never leaves your device.

MKV is a fine container but many phones, smart TVs and editing programs refuse it. Converting to MP4 keeps the video in a far more widely supported format.

Where the video and audio inside are already MP4-compatible the streams are copied rather than re-encoded, which is fast and lossless. Otherwise the file is re-encoded to H.264 and AAC.

Everything happens on your device, so a personal recording is never uploaded to a stranger’s server just to change its container.

How to convert MKV to MP4

Add your MKV file.
Press Convert — where the picture is H.264, HEVC or AV1 and the sound is AAC, MP3, AC-3, E-AC-3 or ALAC, the streams are copied rather than re-encoded, which is fast and lossless.
Download an MP4 that plays on phones, TVs and editors.

Supported formats and limits

Input: MKV (Matroska)
Output: MP4
Speed: Very fast when streams can be copied instead of re-encoded
Quality: Unchanged in the copy case
Subtitles: Subtitle tracks are not carried into the MP4
Where it runs: In your browser; nothing uploaded

Not for: Keeping subtitles — an MKV’s subtitle tracks are not carried into the MP4, so keep the original MKV if you need them. A very large film is handled in parts that must each fit in the device’s memory.

When the streams are copied

The MKV’s streams are copied into the MP4 without being decoded when the picture is H.264, HEVC or AV1, every sound track is AAC, MP3, AC-3, E-AC-3 or ALAC, and Quality and Resolution are left as they are. The picture and every sound track are kept, so a film with several languages keeps them all; a second picture track and subtitles are not carried. This takes seconds, and the note under the result names the picture format that was copied.

An HEVC or AV1 picture copied into an MP4 still needs a player that opens HEVC or AV1. If yours does not, turn on Force re-encoding and the picture is encoded again as H.264.

When the picture is encoded again

Everything else is encoded again as H.264: a VP9 or MPEG-2 picture, DTS, FLAC, Opus or Vorbis sound beside it, a changed Quality or Resolution, or the switch turned on. The result then holds one sound track: the one the MKV marks as its default, or, with none marked, the one with the most channels. AAC, MP3, AC-3, E-AC-3 and ALAC sound is still copied untouched; other sound becomes AAC at 128 kbit/s, in stereo when your device’s own encoder makes the picture. Which encoder does the work is explained at /rotate-video.

Choosing quality and resolution

The Quality slider uses x264’s CRF scale, from 16 to 38, starting at 23. Lower means a better picture and a bigger file. The engine uses the number directly. Your device’s encoder only accepts a bitrate, so the slider value is turned into one from the frame size and frame rate, doubling for every 6 steps down and halving for every 6 steps up, and never above the MKV’s own picture bitrate.

Resolution sets the shorter side of the picture to 1080, 720, 480 or 360 pixels, so a vertical video at 720p becomes 720 pixels wide. A smaller video is never enlarged. To reach an exact file size rather than a quality level, use /compress-video.

Frequently asked questions

Are my files uploaded to a server?: No. This conversion runs inside your browser using your own device’s processing power, and the file is not uploaded. Once a tool has been used once (video, text recognition and background removal download their engine from this site the first time), it keeps working when your connection drops.
Will I lose quality?: Not when the streams can be copied. If a re-encode is needed there is some loss, as with any re-encode, but at default settings it is hard to see.
What about subtitles?: Subtitle tracks embedded in an MKV are not carried into the MP4.