Free converter · Lossy re-encode
Convert MKV to WebM
Short answer
Drop your MKV file into the converter below and download the WebM file. It runs in your browser with FFmpeg, for files up to 100 MB; nothing is uploaded. The file is re-encoded to VP8 video and Opus audio in WebM. Re-encoding takes longer than a container swap and costs some quality; how much depends on the clip and on the quality setting, and at the converter's default setting the loss can be visible, so keep the original.
Maximum file size: 100 MB. The first conversion loads a ~30 MB converter engine, once per visit. Nothing is uploaded - conversion runs on your device.
H.264/AAC re-encoded to VP8/Opus. · Lossy re-encode · Browser-side via FFmpeg WASM - nothing uploaded.
Key takeaways
- The 8.88 MB MKV file was re-encoded and became 925 KB (-89.6%).
- The result holds: Video: vp8 1920x1080; Audio: opus 48000 Hz 2 ch.
- In your browser the same conversion of a 3 second clip took 33.3 seconds on the single-thread engine; natively, the 10 second clip took 8.3 seconds.
- The picture scored SSIM 0.942 against the source (1.0 would be identical).
- Next to it: MKV to AAC (-98.1%), MKV to AVI (-70.6%), MKV to FLAC (-81.6%).
Measured: MKV to WebM
The converter's own ffmpeg arguments, run on a 10 second 1920 × 1080 excerpt of Big Buck Bunny (an animated film, not camera footage) saved in MKV. The browser column is the same arguments on the single-thread WebAssembly core with a 3 second excerpt, in a headless run; your device will differ. Results describe this clip and these encoders.
| Check | Result |
|---|---|
| Mode | Re-encoded |
| Input | MKV, 8.88 MB |
| Output | WebM, 925 KB (-89.6%) |
| Output streams | Video: vp8 1920x1080; Audio: opus 48000 Hz 2 ch |
| Time, native, 10 s clip | 8.3 s |
| Time, browser engine, 3 s clip | 33.3 s |
| Picture similarity (SSIM) | 0.942 |
A re-encode decodes and encodes every frame or sample, so the time grows with the length of the file.
MKV vs WebM
| MKV | WebM | |
|---|---|---|
| Full name | Matroska | WebM (open web format) |
| What is inside | Almost any codec, plus several audio and subtitle tracks | VP8, VP9 or AV1 video with Vorbis or Opus audio |
| Compression | Depends on the codec inside | Lossy |
| Best for | Archiving and playback in desktop players; most social platforms do not list it | Video on web pages; YouTube accepts it, most short-form apps do not |
What you get
VP8 video and Opus audio in WebM. H.264/AAC re-encoded to VP8/Opus.
Where you can upload WebM
| Platform | WebM | What the platform documents |
|---|---|---|
| TikTok | Partly | Yes through the TikTok Content Posting API - native upload support depends on the surface. |
| Instagram Reels | Not documented | WebM is not in Instagram's published video specs, which list MOV and MP4. Convert to MP4 before uploading. |
| Instagram Feed | Not documented | WebM is not in Instagram's published video specs, which list MOV and MP4. Convert to MP4 before uploading. |
| YouTube Shorts | Accepted | Yes - WebM is on YouTube's own list of supported upload formats. |
| X (Twitter) | Not accepted | WebM is not listed for X Media Studio uploads. |
| Facebook Reels | Unclear | Current WEBM support for Facebook video uploads has not been reverified. |
| Not accepted | No - WebM isn't in Pinterest's accepted list on either the app or the web. |
Planning a bigger file
These are estimates scaled up from the 10 second clip. Real files differ with content, resolution and bitrate, so treat them as an order of magnitude, not a promise.
| Length of the video or recording | Input size (like ours) | WebM size | Time in your browser |
|---|---|---|---|
| 1 minute | 53.08 MB | 5.53 MB | 11 minutes |
| 10 minutes | 530.80 MB | 55.32 MB | 111 minutes |
| 1 hour | 3184.83 MB | 331.90 MB | 666 minutes |
An MKV file of this kind reaches the 100 MB limit of the converter after about 2.0 minutes; converting it would take roughly 22 minutes on the single-thread engine, and the WebM would be about 10.93 MB.
MKV and WebM next to their neighbors
The same clip through other conversions with the same source or the same target.
| Conversion | How | Size change | Time (native) |
|---|---|---|---|
| MKV to WebM | Re-encode | -89.6% | 8.3 s |
| MKV to AAC | Re-encode | -98.1% | 0.25 s |
| MKV to AVI | Re-encode | -70.6% | 1.5 s |
| MKV to FLAC | Re-encode | -81.6% | 0.12 s |
| MKV to GIF | Re-encode | +592% | 5.3 s |
| MKV to M4A | Re-encode | -98.1% | 0.27 s |
| MKV to MOV | Copy | +0.1% | 0.10 s |
Choosing WebM
WebM: WebM (open web format). Video on web pages; YouTube accepts it, most short-form apps do not. MKV: Matroska. Archiving and playback in desktop players; most social platforms do not list it.
- Keep your original: a re-encode is a copy of a copy.
- If the destination states the format it accepts, use that and nothing else.
Questions people ask
Is this MKV to WebM converter free and private?
Yes. It is free, needs no account and runs entirely in your browser, so the file never leaves your device.
Will converting MKV to WebM reduce quality?
Some, because WebM is lossy and the file is re-encoded. How much depends on the clip and on the quality setting, and at the converter's default setting the loss can be visible, so keep the original. For uploads, the platform re-encodes again anyway.
Is there a file size limit?
Files up to 100 MB. The first conversion downloads a converter engine of about 30 MB once per visit; larger files are better handled by a desktop app such as HandBrake or FFmpeg.
How do I convert MKV to WebM?
Choose the file in the converter above and download the WebM. It runs in your browser with no upload.
Will converting MKV to WebM reduce quality?
A little. The picture scored SSIM 0.942 against the source in our test.
Why did the file size change by -89.6%?
The target codec and settings decide the size. This 10 second clip came out at 925 KB.
How long does it take?
A 3 second clip took 33.3 seconds in the browser engine in our test; time grows with the length of the file.
How long will an MKV file of 100 MB take?
At the speed we measured in the browser engine, about 22 minutes for a file like our clip. Your device and the content will change it.
Is my file uploaded?
No. The conversion runs in your browser.
Is there a size limit?
Files up to 100 MB. The first conversion downloads a converter engine of about 30 MB once per visit.
More converters
Sources and how we checked
- MDN: Media container formats
- MDN: Web audio codec guide
- MDN: Image file type and format guide
- FFmpeg documentation: stream copy
- FFmpeg Documentation: ffmpeg (stream selection, -map, -c copy) (read 2026-10-09)
- MDN: Media container formats (read 2026-10-09)
- Blender Foundation: Big Buck Bunny (CC BY 3.0), the clip used for the measurements (read 2026-10-09)
Reviewed October 8, 2026. What the converter writes was measured with ffprobe on the same FFmpeg build your browser downloads; format descriptions follow MDN's media guides.