Convert MP4 to WebM
Turn MP4 files into WebM on this device. VP9 or VP8 video and Opus sound, not uploaded.
- Nothing is uploaded
- Works offline after first visit
- Open source
- No watermark · limit is your device’s memory; phones hold less
Converts MP4 videos to WebM. It is free, needs no account and adds no watermark, and your video is not uploaded: your browser reads the file and writes the WebM inside this tab. The picture is re-encoded to VP9 (VP8 where the browser cannot write VP9) and the sound to Opus. The size limit is your device’s memory (phones hold less), and after one visit and a short wait the page works offline.
Done
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Open the network tab. Your files never appear in it.
Add files and run the tool to see every request this tab makes.
Your files are processed on this device and never sent to a server. Reload the page to clear everything from memory.
WebM is the open video format of the web: Chrome, Edge and Firefox play it everywhere, and it is the usual choice for video on a web page, in a game or in an app that wants a royalty-free codec. MP4 is what cameras, phones and editors save, so a file for a web page often has to be converted first.
Worked example. You have talk.mp4, a one-minute clip at 1920×1080 with H.264 video and AAC sound. Drop it and press Convert to WebM. H.264 and AAC cannot go into a WebM, so the picture is re-encoded to VP9 and the sound to Opus by your browser’s own encoders. The line under the result reads "1920×1080, 1:00, re-encoded to VP9", and you download the .webm.
A re-encode takes longer than a copy and, like any lossy re-encode, can cost a little detail; it is done at a high quality setting. A file that is already WebM is copied as it is. A browser with no VP8 or VP9 encoder says so and writes nothing, so use Chrome or Edge on a computer if yours does.
Only the main video and one sound track are kept: extra sound tracks and subtitles are left out. For the other direction, use WebM to MP4; to make a large video smaller, use Compress video.
How it works
- Drop one or more MP4 files, or tap to pick them.
- Tick Remove the sound if you want a silent WebM.
- Press Convert to WebM. One file downloads straight away; several download as a zip.
In your browser, not on a server
Mediabunny (MPL-2.0), a media toolkit written in TypeScript, reads the MP4, MOV, MKV or WebM container in your tab and writes a WebM. VP8, VP9 or AV1 video and Opus or Vorbis sound already in the file are copied across as they are. Anything else is decoded and re-encoded by your browser’s own encoders (WebCodecs), VP9 where the browser can write it and VP8 otherwise, with the sound as Opus. A browser with no VP8 or VP9 encoder says so instead of writing a broken file.
Compared with upload-based tools: The video is read from your disk and the WebM is written in memory on your device, so there is no upload to wait for and the only limit is how much memory your device has.
Verify it yourself. Give the page a short while after one visit to save itself for offline use, then turn off Wi-Fi or switch to airplane mode and run the tool again. Or open your browser's developer tools (F12, or Cmd-Option-I on a Mac), pick the Network tab, and watch it while you convert. The panel above does the same count for you.
Questions
How do I convert MP4 to WebM?
Drop the file on this page and press Convert to WebM. Your browser reads the file and writes a WebM with the same name and a .webm ending. Nothing needs installing, and you can drop several files at once.
VP9 or VP8: which does it write?
VP9 where your browser can encode it, because it gives smaller files at the same quality; VP8 where it cannot. The line under the result names the codec. Sound is always Opus.
Why does my browser say it cannot write WebM?
WebM needs a VP8 or VP9 encoder and an Opus encoder in the browser, and not every browser or Linux build has them. Safari’s engine (WebKit 26.6 on a Mac) writes VP9 and Opus here; older Safari versions may not. The page says so rather than writing a broken file, and Chrome or Edge on a computer write WebM.
Is the video uploaded?
No. Your browser reads the file from your disk and writes the WebM with code running in this tab. Open the network tab, run a conversion, and your video is not among the requests.
Is the file uploaded anywhere?
No. There is no server in this process. The page is a static file and the work is done by code running inside your browser tab. Give the page a short while after one visit to save itself for offline use; then you can turn off Wi-Fi and it still works.
More video and audio tools
Video to WebM
Turn MP4, MOV and MKV files into WebM with VP9 video and Opus sound. Converted on this device, not uploaded.
Trim video
Cut a clip out of a longer video without uploading it. Most cuts take seconds and keep the original quality.
Video splitter
Cut one video into parts at the times you choose, every so often or into equal pieces, and save them in one zip. Done on this device, and the video is not uploaded.
Mute video
Take the sound out of a video, in seconds and without re-encoding. The video never leaves this device.