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WebM to MKV Converter

Turn WebM files into MKV in a couple of clicks. WebM is an Open, royalty-free container built for the web; MKV is a flexible container that holds any codec, subtitles and tracks. Some detail is discarded to fit the target format; how much depends on the format and your settings.

Not uploaded to any serverConverted on this deviceGone when you close the tabHow this works

Drop your files here

WebM files (.webm) · up to 512 MB each

or paste files from your clipboard

Ready when you are

Drop a WebM file above and FileTools360 will turn it into MKV — right here on your device, without uploading anything.

⌘K to search converters

How to use it

1
Add your WebM files
Drag them onto the drop area, choose them from your device, or paste them from your clipboard. You can queue several at once. Nothing is sent anywhere — the file stays on this device.
2
Adjust the settings if you want to
The defaults suit most people. Open Advanced to control codec, resolution and bitrate and the audio track.
3
Convert and download
Press Convert. Everything happens on your own device — nothing is uploaded. Download files one by one, or grab them all as a ZIP.

Privacy, in plain words

Is it safe? Your WebM file is not uploaded anywhere

What “uploaded” normally means. On most online converters, pressing the button copies your file across the internet onto a computer owned by that website. That copy is what the word “upload” means, and it is the part nobody explains.

What happens instead. This site does not do it. There is no server here that receives files, so your WebM file is read, converted and handed back by the browser you are using right now. It never travels — so there is no copy to store, read, leak or hand over.

Check it yourself in ten seconds

Turn off your Wi-Fi after this page has loaded, then convert a file anyway. It still works, because everything needed is already on your device.

The longer explanation of how this works, and the full detail of what is and is not stored on the privacy page.

Why this matters more than it sounds

Video is the format where the thing being converted is usually a recording of people, and often of people who did not expect a third party to have a copy. It matters more than it sounds, because of what video files usually are:

Interviews

sources and participants who consented to you, not to a website

Security-camera footage

your home or premises, and who was in it

Family videos

children, addresses, the inside of your house

Medical or therapy consultations

clinical recordings under strict handling rules

A file that is never sent cannot be kept, leaked, subpoenaed or used to train anything. That is a property of how this works, not a promise about how carefully it is run.

WebM and MKV explained

WebM is an open, royalty-free container built for the web, holding VP8/VP9 or AV1 video with Vorbis or Opus audio. It exists specifically so browsers can ship video support without patent licensing.

Matroska is a container designed to hold essentially anything: any codec, unlimited audio tracks, subtitle tracks, chapters and attachments in one file. It is the format of choice for archiving films with multiple languages.

Where WebM files come from: Web video, and exports from browser-based recording tools.
Where MKV is used: Media archives, anime and film collections, and high-quality recordings.

WebM vs MKV

The two formats solve different problems, which is why converting between them involves trade-offs rather than a straight upgrade.

WebM compared with MKV
Property	Source	Target
Compression	Container (depends on codec)	Container (depends on codec)
Animation	Yes	Yes
Introduced	2010	2002
Browser support	All modern browsers. Weak native OS support.	Great in VLC and media servers, poor in browsers.

WebM is stronger at: Royalty-free with no licensing obligations. Smaller than H.264 MP4 at similar quality. Supported by all modern browsers.
MKV is stronger at: Holds any codec. Unlimited audio, subtitle and chapter tracks. Open and well documented.

Why convert WebM to MKV?

People make this switch when they run into the limits of WebM: sharing files with people who will open them outside a browser. MKV is the better fit when archiving or watching in a capable player such as VLC or a media server.

WebM limitations that prompt the switch: Poor support in desktop players, editors and on TVs. Slower to encode than H.264. Not accepted by many video platforms as an upload format.
What you give up: Browsers will not play it. Many TVs, phones and editors reject it.

Does converting WebM to MKV reduce quality?

MKV compresses without discarding data, but it cannot represent everything a WebM file holds. What does not fit is discarded permanently, so converting the result back will not restore it. Keep the WebM original if it is the only copy.

Measured, not estimated

What converting WEBM to MKV actually costs

On every sample measured, the MKV came out larger — 24% larger at best and 2.6× larger at worst.

Each row is one sample chosen to isolate a property these formats treat differently.
Sample	WEBM in	MKV out	Change
An easy clipThe compressible end of the range — a clean tone, or a moving test pattern — where a codec does its best work.	66.3 KB	82.0 KB	+24%
A hard clipThe other end — pink noise, or dense video grain. Close to incompressible by design, and the worst case for every codec.	323 KB	839 KB	2.6×

Produced by this site's own converter. The same code that runs in your browser, at the same default settings, so these are the bytes you would download.

Samples are generated from a fixed seed rather than taken from real photographs or documents, so the corpus is reproducible and carries no licensing question — and so the numbers predict the shape of your result rather than its exact size. Measured 2026-10-01. The method, and the command that regenerates every figure on this page, are on the how it works page.

About these formats

Source

WebM Video

Open, royalty-free container built for the web.

Extensions: .webm
MIME type: video/webm
Best for: Websites · Open-source projects · Small file sizes
Compatibility: All modern browsers. Weak native OS support.

Target

Matroska Video

Flexible container that holds any codec, subtitles and tracks.

Extensions: .mkv
MIME type: video/x-matroska
Best for: Multi-track video · Archiving · Media servers
Compatibility: Great in VLC and media servers, poor in browsers.

Good to know

WebM to MKV Converter — questions

How do I convert WebM to MKV?

Drop your WebM file onto the area above, then press Convert. The MKV file saves straight to your device. You do not need an account, and there is no watermark.

Is it safe to convert WebM files here — are they uploaded to a server?

No, nothing is uploaded. On most online converters, pressing the button copies your file across the internet onto a computer owned by that website; it is converted there and sent back. That copy is what "upload" means, and it is the part that puts your file out of your control. This site does not do it. Your WebM file is read and converted by the browser you are using right now and never leaves this device, so there is no copy for anyone to store, read or hand over. You can prove it: turn off your Wi-Fi after this page loads and the conversion still works.

Can I convert confidential WebM files, like bank statements or contracts?

Yes, and that is the case this site was built for. Because the file never leaves your device, a confidential document is no more exposed than it is sitting in your own folder — there is no transfer, no copy on someone else's hardware, no retention period to read about and no staff with access. Nothing is logged about the file either: not its name, not its size, not its contents. The one thing worth saying plainly is that this protects the file in transit and at rest on our side, not from anything already on your own computer.

Does converting to MKV reduce quality?

Yes, though not through compression — MKV stores what it holds without discarding data, but it cannot hold everything a WebM file does. What does not fit is dropped permanently, and converting the result back will not bring it back.

How large a file can I convert?

Up to 512 MB per file for this conversion. Because the work happens in your browser, the practical limit is your device's memory — larger files work on a desktop with plenty of RAM and may fail on an older phone.

Can I convert several files at once?

Yes. Add as many as you like and press Convert All. Each file is processed independently, so one failure does not stop the rest, and you can download everything together as a ZIP.

Can I use this on my phone?

Yes. FileTools360 works in mobile browsers on iOS and Android, including choosing files straight from your camera roll. Very large videos may run out of memory on a phone — those are better converted on a laptop.