WebM to MP4 converter
Re-encode a WebM to H.264 and AAC MP4 that iPhones, editors and upload forms accept.
Runs in your browser. Nothing is uploaded.
Drop a video
MOV, WebM, MKV or any video your browser can read. It stays on your computer.
More about this tool
- 1
Add a WebM
Drop the file. The tool shows which video and audio it holds.
- 2
Convert
The video is encoded to H.264 and the sound to AAC. A long recording takes a while, so keep the tab open.
- 3
Check and download
Play the MP4 to check it, then download it.
WebM is the web's open format. Its video is VP8, VP9 or AV1 and its audio Opus or Vorbis. Browsers play it, but many video editors, upload forms and phone apps don't, and moving VP9 into an MP4 box doesn't help them.
So unlike MOV, a WebM can't just be moved into MP4: the video is decoded and encoded again as H.264, and the sound as AAC. That's the pair nearly every device and app takes. Re-encoding at high quality keeps the picture close to the original, but it takes time and the file can come out larger than the WebM.
Screen recorders that run in a browser usually save WebM, the format browsers' recording API has long written. Those files often have no length in their header, which is why some players show no duration or can't seek. The converted MP4 has its length and index written, so it seeks normally.
The same conversion on the command line. -crf 20 sets the quality (lower is better), and +faststart puts the index first:
- ffmpeg -i input.webm -c:v libx264 -crf 20 -c:a aac -movflags +faststart output.mp4
Sources
Questions
No. Your browser decodes the WebM and encodes the MP4 itself. Nothing is sent anywhere.
It depends on the video's length and resolution and on your computer, since every frame is encoded. A short clip takes seconds; a long 1080p recording can take several minutes. The progress shows how far it is.
A little, as any re-encoding does. It's set to high quality, so the difference is hard to see, but the MP4 can be larger than the WebM.
Chrome and Edge decode and encode the most formats. If the tool says your browser can't encode H.264 or AAC, try one of them.
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