Convert WebM to MP4
Convert WebM screen recordings, OBS captures and Discord clips to MP4 with real VP9 to H.264 transcoding, determinate progress and no length cap.
- Runs in your browser
- Files are never uploaded
- Free, no account
First use downloads the 10 MB video engine from this site. After that it is cached and works offline. It is WebAssembly served from onthisdevice.com — no third-party CDN — and your file is still processed entirely on your own device.
Why WebM to MP4 is real work
Most container conversions on this site are a copy: the compressed data already fits the destination and only the wrapper changes. WebM is the exception. It holds VP8 or VP9 video and Opus audio, none of which an MP4 can carry in a form that players understand. Every frame must be decoded back to pixels and re-encoded as H.264, and every audio sample re-encoded as AAC. There is no shortcut, and any service that returns your file instantly has simply renamed it — the result will fail on the device you were trying to use it on.
The upside of doing that work here is that the encode runs on your processor rather than someone’s rented one, so there is no queue, no size cap, no watermark on the output and no copy of your screen recording left behind on a server.
Roughly how long it takes
| Clip length | 720p screen recording | 1080p camera footage |
|---|---|---|
| 30 seconds | 15-40 seconds | 30-60 seconds |
| 5 minutes | 3-7 minutes | 5-12 minutes |
| 30 minutes | 20-45 minutes | 30 minutes to well over an hour |
Those are honest laptop numbers, not marketing ones. Two things speed them up a lot: choosing the small file quality preset, and choosing a faster encoder preset under advanced options. Screen recordings in particular compress extremely well because most of the frame does not change between frames, so the cheaper settings cost very little visible quality.
The seeking bug this quietly fixes
Anything recorded in a browser — by our own screen recorder, by Loom-style web apps, by a WebRTC call recorder — comes out of the MediaRecorder API with no duration written in the header. The file is complete and plays fine from the start, but the player has no idea how long it is, so the scrub bar shows nothing useful and dragging it does not work. Re-encoding writes a proper header, and the timeline behaves again. That alone is a good reason to convert a browser recording before you send it to anyone.
Where WebM files come from
- OBS Studio and other streaming tools, when set to a WebM output.
- Discord clips and downloads.
- Browser screen and camera recorders, which record WebM natively.
- Saved web video — YouTube and many sites serve VP9 in WebM to browsers that support it.
- Telegram and WhatsApp stickers, which are short VP9 WebM animations.
Audio: what to expect
Opus is a better codec than AAC at the same bitrate, so re-encoding it costs a little quality in principle. At the 160 kbps AAC this page writes, the difference is not audible on speech or on typical music. If the result is completely silent, the source had no audio track at all — overwhelmingly the most common cause is a screen recording started without the “share audio” option ticked, which cannot be recovered after the fact.
The reverse direction
Going the other way, to a smaller file for a website, is MP4 to WebM. If you only need the sound from the recording, WebM to MP3 skips the video work entirely and finishes in a fraction of the time.
Questions
Why is WebM to MP4 slower than MKV to MP4?
Because it is a genuine transcode. WebM holds VP8 or VP9 video, which MP4 cannot legally carry in a way that players understand, so every frame has to be decoded and re-encoded as H.264. There is no shortcut — any site that returns a WebM to MP4 conversion instantly has simply renamed the file.
How long will a 10-minute screen recording take?
Roughly 8-15 minutes on a modern laptop at the default settings, and you can watch the real percentage climb. Screen recordings compress unusually well because most of the frame is static, so choosing “small file” costs very little visible quality and finishes noticeably faster.
My WebM has no sound after converting — why?
WebM audio is Opus or Vorbis, and both are re-encoded to AAC here. If the result is silent, the source recording had no audio track at all, which is common when a screen recorder was started without the “share audio” checkbox ticked.
Why will my browser-recorded WebM not seek?
MediaRecorder writes WebM files with no duration in the header, so players cannot scrub them. Converting rebuilds the file properly and the timeline works again — that alone is a good reason to convert a browser recording before sharing it.