Convert WebM to MP4
Your files are processed on your device. We never upload them.
Video is read and written with Mediabunny, under the Mozilla Public License. Browsers without their own AAC encoder use FFmpeg's, under the GNU LGPL.
How to convert WebM to MP4
- 01A ▸Choose your WebM videos, or drop them on the page.
- 02A ▸Check what the page says under each video. Re-encoding takes a while.
- 03A ▸Select Convert to MP4, wait for the progress bar, then download each video.
About WebM to MP4
Where WebM files come from
WebM is the web's own video format, written by screen recorders, browser-based recorders, and many downloads. Older iPhones and Macs, TVs, PowerPoint, and video editors often can't open it, but they all open MP4 with H.264.
What changes
- The picture is re-encoded from VP8, VP9, or AV1 to H.264, and the sound from Opus or Vorbis to AAC, at a high quality setting.
- Size and frame rate stay the same. A video with a see-through background loses its transparency, because H.264 has none.
- The file size changes; H.264 is usually somewhat larger than VP9 at the same look.
Measured
Our one-minute 1080p sample at 30 frames a second with VP9 video and Opus sound (a synthetic test pattern, which is easier to encode than real footage), converted in Chrome on a laptop. Phone timings are still to be measured.
| WebM file | 33.4 MB |
|---|---|
| MP4 file (H.264 and AAC) | 47.1 MB |
| Time (Chrome, laptop) | 5.4 seconds; real footage takes longer |
| Time on phones | still to be measured |
In your browser
Needs the browser's own video and audio encoders (WebCodecs). Chrome, Edge, and Firefox have them; Safari has video from 16.4 and audio (needed for the sound) from 26. Where an encoder is missing, the page says so before you start. On computers the browser usually uses the graphics chip, which is much faster than software.
Limits
- A long or 4K WebM can take many minutes on a phone; a computer is faster. On a computer, the page shows roughly how long before you start.
- Transparency is lost; for a see-through video, keep it as WebM.
Other options
- MP4 to WebM: the reverse, for smaller web videos.
- MKV to MP4: OBS recordings saved as MKV, often converted in seconds.
- Video converter: convert to MOV or MKV instead.
Questions
Why does WebM to MP4 take longer than MOV to MP4?
WebM holds VP8, VP9, or AV1 video and Opus or Vorbis sound, which most MP4 players don't expect. So the picture is re-encoded to H.264 and the sound to AAC, frame by frame, with your device's own video encoder. Expect it to take a good fraction of the video's length on a computer, and longer on a phone.
Are my videos uploaded?
No. Your files are processed on your device. We never upload them.
Does it lose quality?
Re-encoding always makes a new copy, but at the high quality setting used here the difference is hard to see. The file may be larger or smaller than the WebM.
Can I convert on my phone?
Yes, if its browser has WebCodecs, as Chrome on Android and Safari 26 or later on iPhone (earlier versions can't convert the sound). Long or 4K videos take a long time on a phone; the progress bar shows the time left. A computer is faster.
Where do WebM files come from?
Screen recorders, browser recordings, video downloads, and sites such as Discord and Reddit save WebM because browsers encode it easily.