WEBM to MKV Converter
FileFlip converts WEBM to MKV entirely inside your browser, using WebCodecs and FFmpeg compiled to WebAssembly. The file is never uploaded to a server: it is read off your disk, converted on your own machine, and saved back by the browser. That means no file-size limit, no queue and no account.
How do I convert a WEBM file to MKV?#
- Drop your WEBM file onto the converter at the top of this page, or click it to pick one from your computer. You can add as many as you like.
- Leave the target set to MKV and the conversion starts on its own. FileFlip usually downloads nothing, falling back to 30.8 MB of WebAssembly if your browser cannot encode these codecs itself, then runs WebCodecs and FFmpeg on your own machine.
- Save the MKV file. Your browser writes it straight to your downloads folder, because there was never a server holding it.
Does converting WEBM to MKV lose quality?#
Sometimes. When the codecs inside the WEBM file are ones the MKV container can hold, the streams are copied straight across and nothing at all is lost. When they are not, the file has to be decoded and encoded again, and that step does cost a little quality.
Is it safe to convert WEBM files online?#
Yes, because nothing is uploaded. FileFlip converts WEBM files inside the browser tab you already have open, using WebAssembly builds of the same engines a desktop converter would install. Your file is read from your own disk into the page's memory, and no request carrying it is ever made.
You do not have to take that on trust. Open your browser's network panel, run a conversion, and you will see the engine come down and your file go nowhere. There is no upload, no server-side copy, no retention window, and no account tied to what you converted.
How FileFlip works lists every engine, its version and its download size, and shows how to check for yourself that nothing leaves your machine.
What WEBM to MKV actually does to the file#
Converting WEBM to MKV runs on WebCodecs and FFmpeg, usually downloads nothing, falling back to 30.8 MB of WebAssembly if your browser cannot encode these codecs itself, copies the streams through when the codecs fit and re-encodes when they do not.
| Engine | WebCodecs and FFmpeg |
|---|---|
| Conversion path | One step: WEBM → MKV |
| Downloaded to your browser | Nothing in most browsers, or 30.8 MB if yours cannot encode these codecs itself |
| Quality | Lossless when the codecs fit, lossy when they do not |
| You get back | One file |
| Where it runs | In a background worker, so the page stays responsive |
What survives the conversion
| What is in the file | This conversion | Why |
|---|---|---|
| Encoded streams | Sometimes | Survives for some files and not others. |
Worth knowing before you start:
- the FFmpeg fallback is an LGPL build, so a re-encode produces MPEG-4 Part 2, VP8, VP9, or Theora rather than H.264 or H.265
- whether the browser can do this without FFmpeg depends on the codecs it exposes through WebCodecs
- the encoded streams are copied when the file's codecs fit the target container, and re-encoded when they do not
What is a WEBM file?#
A WEBM file holds VP8, VP9 or AV1 video with Vorbis or Opus audio in a cut-down version of the Matroska container, restricted to codecs that carry no patent licensing so browsers can play them freely.
Choose WebM when the video is going on a web page and you want it to play in browsers without a codec licence attached to it.
Convert WebM to MP4 when the file has to leave the browser, whether that is an edit timeline, an older iPhone or a television.
What is an MKV file?#
An MKV file is a Matroska container built on the EBML binary format, and it can hold any number of video, audio, subtitle and attachment tracks, encoded with essentially any codec, in a single file.
Use MKV when a video needs multiple audio tracks, several subtitle languages, or a codec MP4 doesn't accommodate, and it's staying on software you control, like a media server or a desktop player.
Convert MKV to MP4 when a file needs to play on a device that never added MKV support, since that's usually a hardware or software gap rather than anything wrong with the codec inside.
WEBM to MKV questions people ask#
What is the difference between WebM and MKV?
WebM is a deliberately narrowed version of Matroska. It uses the same container structure but only allows VP8, VP9 or AV1 video with Vorbis or Opus audio, where MKV takes any codec at all. That restriction is the point: a browser can support WebM completely, whereas supporting MKV would mean supporting every codec ever put in one.
Why did my screen recording save as a WEBM file?
Browser-based recorders and screen capture tools save WEBM because it is what the browser can encode without licensing anything. Recording in the browser goes through the MediaRecorder API, and VP8 or VP9 in WebM is the combination browsers ship by default, since encoding H.264 would mean paying for the patents. The recording is fine, it just needs converting before most editing software will take it.
Does converting WebM to MP4 lose quality?
Converting WebM to MP4 usually re-encodes the video and loses a little quality, because the VP9 or VP8 inside a WebM is not what MP4 players expect to find. MP4 can technically carry VP9 and AV1 now, but almost nothing in the wild plays those from an MP4, so a real conversion means encoding to H.264. Going the other way, MP4 to WebM, always re-encodes too, since H.264 is not allowed inside WebM at all.
Can WebM video have a transparent background?
WebM is one of the few video formats that carries transparency, using an alpha channel alongside VP9. That makes it the usual way to put video over a web page without a rectangular black box around it. The transparency does not survive conversion to MP4, since H.264 in MP4 has no alpha channel to put it in.