Convert WebM to MP4 Online — No Upload, No Watermark
WebM is the open royalty-free container used by YouTube and Chrome. Converting to MP4 gives you a file that plays on every phone, browser, and editor without re-encoding at playback time.
Drag & drop your video here, or click to browse
Max file size: ~2 GB (memory permitting)
Where WebM files come from
WebM comes out of browser-based recorders. Anything captured with MediaRecorder, including Loom-style screen tools, OBS browser output and this site's own screen recorder, lands as VP8 or VP9 video with Opus audio.
What changes when you convert WebM to MP4
| Video | VP8 or VP9 re-encoded to H.264 |
|---|---|
| Audio | Opus re-encoded to AAC |
| Duration metadata | Written properly, which browser recordings frequently lack |
Worth knowing first
Browser recordings often report a duration of zero or Infinity because MediaRecorder writes the header before it knows the length. Editors then show the clip as unseekable. Converting to MP4 rewrites the header with a real duration, which is usually the actual reason a WebM will not scrub in Premiere or Resolve.
FAQ
Why will my screen recording not scrub in my editor?
MediaRecorder writes the WebM header before the recording ends, so the duration field is often zero or Infinity. Most editors refuse to build a timeline from that. The MP4 written here carries a correct duration, which fixes seeking without you having to re-record.
Is VP9 to H.264 a downgrade?
At the same file size VP9 is the more efficient codec, so a straight transcode does lose a little. At CRF 23 the difference is invisible on screen recordings and talking-head footage, which is what most WebM files contain. The gain is that the result plays in editors and on iPhones, which VP9 in WebM does not.
Does the alpha channel survive?
No. VP8 and VP9 can carry transparency and H.264 cannot. If your WebM has a transparent background, converting to MP4 flattens it onto black. Keep the WebM for that case, or export a ProRes file from an editor instead.
Is this WebM to MP4 converter really free and without upload?
Yes. The tool runs entirely in your browser using FFmpeg compiled to WebAssembly. Your WebM file never leaves your device — open the DevTools Network tab to verify zero uploads. No signup and no watermark. The practical size limit is the memory your browser gives this tab, which is generous on a computer and a few hundred megabytes on a phone.