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Why this runs on your computer

There is no upload: The work happens in this tab. Your file never travels, so there is no copy of it anywhere but your own disk.
Nothing to delete later: Sites that upload promise to delete your file afterwards. That promise only matters because the file was theirs to hold.
It works with the Wi-Fi off: The honest test. Turn off your connection and the tools keep working, because everything they need is already here.

Drop your file hereto load the tools

Nothing is uploaded — the tools run right here

Applies to every loaded image

Look Strength 100% Brightness Contrast Saturation Vibrance Vignette

Max width Format Shape

Shape Size Across Down Border

Text Where

100%

Convert WebM to MKV without uploading it

Turn a WebM file into MKV — here in this tab, with nothing uploaded.

How to convert WebM to MKV

Drop your WebM file in.
Choose MKV as the output.
Save the MKV file.

Why this one stays private

Converting a video elsewhere means uploading the whole thing, waiting, and trusting a stranger's server with footage that may not be yours to hand over. The conversion happens here instead.

Questions

Why convert WebM to MKV?

WebM is small and made for the web, though older players and editors ignore it. MKV is the archivist's container: it holds anything, but few devices play it directly. Converting is how you move a file from the first situation to the second.

Does converting lose quality?

Re-encoding always costs something. Going to a modern format at a generous bitrate costs very little; going through several conversions in a row is what does real damage.

Which format should I pick?

MP4 if you are not sure — it plays everywhere. WebM if it is for a web page and you want it small.

How to convert WebM to MKV

Why this one stays private

Questions

The same tool, other formats