Convert MKV to WebM
To convert MKV to WebM, add your .mkv file below and press the button: it's processed on your own device and you download the result, with nothing uploaded. It's free, needs no account, and works with files up to 500MB.
About converting MKV to WebM
MKV (.mkv) files can be converted to WebM entirely in your browser — no software to install, no file upload, and no account required. The conversion runs on a WebAssembly build of ffmpeg directly on your device.
Technical comparison
| MKV | WebM | |
|---|---|---|
| Category | video | video |
| Compression type | Varies (codec-dependent — a container, not a codec) | Lossy (VP8/VP9/AV1 video, Vorbis/Opus audio) |
| Typical file size | Varies — depends entirely on the codecs it wraps | Small to moderate — typically smaller than an equivalent MP4 at the same quality |
| Transparency support | Depends on the wrapped codec | Yes (VP8/VP9 support an alpha channel) |
How to convert MKV to WebM privately
Every step below runs on a WebAssembly build of ffmpeg inside this browser tab. Your .mkv file is never uploaded to a server — there's no network request carrying it anywhere.
Add your MKV file
Drag and drop, or click to select, a file (.mkv). It's read straight from disk into browser memory — no upload step exists.
Convert locally
FreePrivateConvert converts it to WebM using a WebAssembly build of ffmpeg, entirely within this browser tab.
Download the result
Save your converted .webm file. It only ever existed in this tab's memory — closing the tab without downloading discards it completely.
Convert MKV to WebM with ffmpeg (command line)
Prefer the terminal? This is the exact ffmpeg command this page runs for you:
ffmpeg -i input.mkv -c:v libvpx -crf 10 -b:v 2M -c:a libopus output.webm- -i input.mkv
- Your MKV file.
- -c:v libvpx
- Encode the video as VP8, the original WebM video codec, supported by every WebM player.
- -crf 10
- Quality level for VP8 (4-63, lower = better quality; 10 is high quality). Works with -b:v below as a bitrate ceiling.
- -b:v 2M
- Maximum video bitrate of 2Mbit/s, so busy scenes can't balloon the file size.
- -c:a libopus
- Encode the audio as Opus, the standard audio codec for WebM.
- output.webm
- The WebM file to create. ffmpeg picks the container from this extension.
Don't have ffmpeg installed? You don't need it: use the converter at the top of this page. It runs the same conversion on your own device, and your file is never uploaded.
FAQ
- Is my file uploaded anywhere?
- No. Your file is read and processed inside this browser tab and never leaves your device. You can confirm it in your browser's developer tools: the Network tab shows no upload.
- Does converting MKV to WebM reduce quality?
- The video is re-encoded as VP8 (CRF 10, capped at 2 Mbit/s) with Opus audio. Quality is good for most clips, but the bitrate cap can soften high-resolution or fast-moving video.
- Is there a file size limit?
- Yes: 500MB per file. There's no account and no limit on how many files you convert.
- What's the difference between MKV and WebM?
- MKV (Matroska Video) and WebM store data differently. MKV: varies (codec-dependent — a container, not a codec). WebM: lossy (VP8/VP9/AV1 video, Vorbis/Opus audio). WebM is an open, royalty-free video container built specifically for the web, pairing VP8/VP9/AV1 video with Vorbis/Opus audio.
- Why convert MKV to WebM?
- Common uses for WebM: Web video without codec licensing costs; Bandwidth-constrained streaming; Progressive web apps. Open, royalty-free codec stack with no licensing fees, unlike H.264/HEVC.
- What MIME type does a .webm file use?
- video/webm, the standard type for WebM.
- What is the ffmpeg command to convert MKV to WebM?
- Run: ffmpeg -i input.mkv -c:v libvpx -crf 10 -b:v 2M -c:a libopus output.webm. This is the exact command FreePrivateConvert runs for this conversion, so you can use this page instead of installing ffmpeg: it runs the same conversion inside your browser.
Learn more: What is an MKV file
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