WebM files: made for browsers, awkward everywhere else
What WebM contains, why screen recorders produce it, and why the MP4 you convert it to may be larger. Convert WebM in your browser, no upload.
At a glance
- Type
- Video
- Container format
- Yes
- Extensions
- .webm
- MIME type
- video/webm
- Common codecs
- vp8, vp9, av1, opus, vorbis
WebM is a Matroska container trimmed down to a short list of royalty-free codecs: VP8 or VP9 video with Vorbis or Opus audio. Google published it in 2010 so the web could have video without licensing negotiations, and it worked — every browser plays WebM, and browser APIs that record video or audio produce it by default.
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That last point is why most people encounter the format. A screen recorder running in a browser tab, a meeting capture, a voice note from a web app, a download from a site that serves WebM: all of it arrives as .webm. Then you try to open it in an editor and discover that the world outside the browser is built around H.264 — because that is what cameras produce and what hardware decodes.
Converting to MP4 is a genuine re-encode rather than a repack, since VP9 and H.264 are unrelated codecs, so it takes a noticeable fraction of the clip's length. And the result is frequently larger than the WebM despite losing a little quality, because VP9 is simply the more efficient codec. Screen recordings show this most sharply: they are mostly static, which is exactly what VP9 compresses best.
One asymmetry to know about on this site: the WebM we produce is VP8 with Vorbis rather than VP9 with Opus. In the WebAssembly ffmpeg build shipped here, the VP9 encoder crashes on the first frame and the Opus encoder fails on every input, while both decoders work fine. So conversions out of WebM are unaffected, and conversions into it are a generation behind and single-threaded.
Convert WebM to other formats
Convert other formats to WebM
FAQ
Why is my converted MP4 bigger than the WebM?
Because VP9 compresses more efficiently than H.264, especially on screen recordings and animation. You are converting for compatibility and paying for it in both size and a little quality.
Why does my editor refuse WebM?
Because editing software is built around the codecs cameras produce — H.264 and HEVC. VP8 and VP9 came from the web rather than the camera industry, so support outside browsers stayed patchy.
How long does converting to MP4 take?
A noticeable fraction of the clip's own duration, because every frame is decoded and re-encoded. This is not the seconds-long repack that MOV to MP4 gets.
Can I convert audio out of a WebM?
Yes, and it is fast — the video is discarded rather than decoded. The audio inside is usually Opus, so give the MP3 a generous bitrate: Opus at 32 kbps sounds fine, MP3 at 32 kbps does not.
Why does this site produce VP8 instead of VP9?
Because in the WebAssembly ffmpeg build used here, the VP9 encoder crashes on the first frame and the Opus encoder fails on every input. VP8 with Vorbis is the combination that works, and every browser still plays it.
Is WebM better than MP4?
Technically the codecs inside are more efficient, and it carries no patent licensing. MP4 plays on far more devices. Which is better depends entirely on where the file is going.
Do I need WebM for a website?
Not for playback — every browser plays MP4 too. Choose WebM to avoid patent-encumbered codecs, or serve both and let the browser pick.
Does audio-only WebM work here?
Yes. Browser voice recorders often produce a WebM with no video track; that converts the same way and slightly faster.