Compress WebM video for the web, with VP9 or AV1
This WebM compressor re-encodes WebM files — or any MP4, MOV or MKV you want as WebM — into a smaller WebM with VP9 or AV1 video and Opus audio. Give it a target size in MB or a quality level and it does the rest in your browser, so recordings of your screen, meetings or browser tabs never leave your computer.
WebM is the open, royalty-free format of the web. It is what browser-based screen recorders and the MediaRecorder API usually produce, and those recordings are written in real time, favouring speed over size, so they often come out larger than they need to be. Re-encoding them with a proper bitrate usually shrinks them a lot, and the new file gets a full duration and seek index, so players can jump to any point. Encoding uses WebCodecs through the Mediabunny library (MPL-2.0).
How to use the WebM Compressor
- Drop the WebM file on the box or click to choose it. The tool shows the codec (VP8, VP9 or AV1), resolution, frame rate, duration and audio.
- Pick Target file size and enter the MB you need, or Quality and move the slider.
- Choose VP9 for speed and compatibility or AV1 for the smallest file, and set the Audio bitrate (Opus).
- For screen recordings, a lower Frame rate such as 15 or 24 fps saves a lot when little moves.
- Press Compress video, compare the two versions and press Download WebM.
WebM Compressor features
VP9 or AV1 video
Both are open codecs. VP9 encodes faster and plays almost everywhere; AV1 usually gives the smallest file at the same quality.
Opus audio
Opus audio is copied as it is when you keep the original; anything else is converted to Opus at the bitrate you choose.
Seekable output
The new WebM has a duration, a seek head and cue points — browser recordings often lack them.
Size target with retry
If a pass lands over your target, the bitrate is corrected from the measured result and the video is encoded again.
Any input
MP4, MOV and MKV files can be compressed into WebM too. Formats browsers can’t decode, such as AVI or WMV, are decoded first with ffmpeg.wasm (GPL-2.0-or-later) after you agree to a 32 MB download.
VP9 or AV1?
WebM can hold VP8, VP9 or AV1 video with Vorbis or Opus audio. This tool writes VP9 or AV1 with Opus, the modern combinations. The bitrate figures are the rule of thumb its quality mode uses, relative to H.264.
| VP9 | AV1 | |
|---|---|---|
| Bitrate for similar quality | About 65% of H.264 | About 50% of H.264 |
| Encoding speed in the browser | Fast | Slower, especially at high resolutions |
| Playback | Chrome, Edge, Firefox, current Safari, Android | Current Chrome, Edge, Firefox, Android; Apple devices with AV1 hardware |
| Good for | Everyday web video, quick turnarounds | Smallest files, long-term storage |
Choosing an Opus bitrate
Opus is very efficient. The Opus project’s recommended-settings page (wiki.xiph.org) suggests 10–24 kbps for mono speech, 64–96 kbps for streaming stereo music and 96–128 kbps for storing it, and calls 128 kbps VBR “pretty much transparent”. For a screen recording with narration, 48–64 kbps is plenty; for a music video, keep 128 kbps or more. When the target size is tight, every kilobit you take from the audio goes to the picture.
Getting the most out of screen recordings
- Lower the frame rate. Slides, code and documents barely move; 15 fps keeps them readable at a fraction of the bits.
- Keep the resolution for text. Small text turns to mush when downscaled; if you must save space, lower the frame rate before the resolution.
- Cut the dead time. Trim the start and end with Trim Video before compressing.
- Need a looping preview? A short WebM is usually far smaller than the same clip as a GIF; make a GIF with Video to GIF only where GIF is required.
- Embedding on a site? Offer an MP4 alongside the WebM for older players; the MP4 compressor makes one.
WebM Compressor FAQ
How do I compress a WebM file?
Drop it on the WebM compressor, choose a target size or quality, pick VP9 or AV1 and press Compress video. The new WebM downloads when you press Download WebM.
Why are WebM screen recordings so large?
Recorders encode in real time and usually aim for a fixed, generous bitrate, regardless of how little changes on screen. Re-encoding with a sensible bitrate and frame rate removes that waste.
My WebM has no duration and can’t be seeked. Will this fix it?
Yes. The compressed file is written with a duration, a seek head and cue points, so players show its length and can jump to any position.
Can I convert an MP4 to WebM while compressing?
Yes. Drop an MP4, MOV or MKV and the result is a WebM with VP9 or AV1 video and Opus audio.
Does WebM play on iPhones and Macs?
Current Safari plays WebM with VP9 and Opus. AV1 needs a device with AV1 hardware. If you need guaranteed playback on older Apple devices, use MP4 with H.264 instead.
Is this WebM compressor private?
Yes. Everything runs in your browser; your recording is never uploaded, and only the Mediabunny library is downloaded from a CDN.