Convert to and from MKV, free.
MKV (Matroska) is an open container that can hold any codec plus unlimited audio tracks, subtitle tracks and chapters. That flexibility makes it great for archives and useless for sharing.
Convert something else to MKV
Pick what you have. Each page opens the editor with MKV already selected as the output.
Convert MKV to something else
Pick where it is going. Browsers, phones and social uploads reject MKV almost universally.
What MKV actually is
Matroska
Almost anything: H.264, HEVC, AV1, VP9
Archiving with multiple audio and subtitle tracks
VLC and desktop players; rarely anywhere else
Depends entirely on the codec inside
Keeps multiple audio and subtitle tracks in one file.
Browsers, phones and social uploads reject MKV almost universally.
Editing tools often refuse to import it, or import picture without audio.
Nobody can preview it without installing VLC first.
Questions about MKV
What is an MKV file?
MKV (Matroska) is an open container that can hold any codec plus unlimited audio tracks, subtitle tracks and chapters. That flexibility makes it great for archives and useless for sharing. It is the Matroska container, usually holding Almost anything: H.264, HEVC, AV1, VP9.
Is converting MKV free?
Yes. Convert and preview for free, then create a free account to download the result. Nothing is installed and there is no watermark on the file itself.
Will converting to MKV lose quality?
Some, whenever the codec changes: every re-encode discards detail. Where the container can simply be rewritten around the existing streams it is lossless, and where it cannot the default settings are chosen so the loss is not visible at normal viewing size.
Other formats
Converters
Convert it, then keep editing.
Every converter opens the full editor, so you can trim, caption and share without uploading the file twice.