Is WebM to MKV lossless?

Yes, in the fullest sense: because WebM is a subset of Matroska, the compressed streams are copied into the .mkv without being decoded, so the video that comes out is the video that went in. There is no quality decision to make on a stream copy, and the size barely moves.

Why do some tools take .mkv but reject .webm?

Accepted-format lists are often written by extension, and .webm is a younger, web-associated name that many lists simply omit, even where the underlying decoder handles VP9 fine. Rewrapping to MKV clears that gate without altering the stream. If a tool fails on the MKV too, it lacks VP9 support, and an H.264 MP4 is the fix.

Is MKV or MP4 the better target for a browser recording?

Different jobs. MKV preserves the original VP9 untouched, so it suits archiving and playback in capable players like VLC. MP4 requires re-encoding to H.264 but opens in editors, phones, and upload forms. A common pattern: keep an MKV master and produce MP4s from the WebM when distribution calls for them.

Is my video uploaded to a server?

No. The whole conversion runs in your browser. Your video is never sent to or stored on a server, so your privacy is preserved.

Can I convert large or long videos?

You can, but because everything runs in your browser, large/long videos and conversions that need re-encoding take time. We recommend files up to around 200MB.

Yes, it is completely free. No sign-up and no installation required.

Embed

WebM to MKV converter

Video tools

Convert WebM videos to MKV right in your browser — no upload, no watermark. When the codecs match it's a lossless remux; otherwise it re-encodes locally. Your video never leaves your device.

What is this WebM to MKV converter?

A free, browser-based tool that converts a WebM video to MKV. Add a file, preview it inline, and download the result. All processing happens locally in your browser, so your video is never uploaded and there is no watermark.

WebM is secretly Matroska, so MKV takes it as-is

Here is the technical fact that makes this pair special: WebM is a constrained subset of Matroska, defined in 2010 by taking the MKV container and restricting it to VP8, VP9, and AV1 video. MKV therefore holds a WebM's streams natively, and the conversion is a remux: the compressed video and audio are copied into the .mkv unchanged. It completes in moments, the output is pixel-identical to the source, and the size stays essentially the same.

That makes this the right move when the point is preservation: turning a pile of browser-recorded webinars, user tests, or meeting captures into archive files without spending a single generation of quality on the transition.

What the .mkv extension buys you, and what it cannot

Plenty of software judges a file by its extension before looking inside. Media managers, intake pipelines, and desktop players that happily handle Matroska often leave .webm off their accepted list, so the same bits get through once they wear the .mkv name. VLC, mpv, Plex-style servers, and ffmpeg-based pipelines all treat VP9-in-MKV as a first-class citizen.

The limit to keep in mind: the codec inside is still VP9. An app with no VP9 decoder (QuickTime, iMovie, most phone galleries) will refuse the MKV just as it refused the WebM. When the destination is an editor, a phone, or an upload form, the answer is an H.264 MP4, not a smarter wrapper.

How to use it

Add a WebM file on the left (drag & drop or click) — it shows an inline preview.
Pick a quality preset (used only when the video has to be re-encoded).
Press “Convert & download” to get your MKV file. The first run downloads the ~30MB converter once.

Lossless remux vs re-encoding

WebM and MKV are containers — what matters is the video codec inside. When the codec already fits MKV, this tool just repackages it (a "remux"): instant and completely lossless. Only when the codec isn't compatible — most often anything involving WebM — does it re-encode the video locally, which takes longer and is where the quality preset applies.

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