# WebM to MP4 converter: free, in your browser, no upload

> Convert WebM to MP4 in your browser. VP8, VP9 or AV1 video and Opus audio are encoded to H.264 and AAC, the MP4 that iPhones, video editors and upload forms all take. Nothing is uploaded.

URL: https://cutfac.com/tools/webm-to-mp4  
Last updated: 2026-10-28  
Language: [한국어](https://cutfac.com/ko/tools/webm-to-mp4)

Drop in a WebM and get an H.264 MP4 that plays on an iPhone and opens in any editor. It's encoded in your browser, so the file is never uploaded.

Runs in your browser. Nothing is uploaded.

## How to use it

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.

## Why WebM needs re-encoding

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.

## Where WebM comes from

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.

## With ffmpeg

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

- [MDN: MediaRecorder](https://developer.mozilla.org/en-US/docs/Web/API/MediaRecorder)
- [FFmpeg documentation](https://ffmpeg.org/ffmpeg.html)

## Questions

### Is my video uploaded?

No. Your browser decodes the WebM and encodes the MP4 itself. Nothing is sent anywhere.

### How long does it take?

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.

### Does it lose quality?

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.

### Which browser works best?

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.