WebM to MP4 Converter
Convert WebM videos (VP8, VP9, screen recordings) to MP4 with H.264 in your browser. Fixes variable frame rate recordings; choose quality and size.
WebM is the open video format used by browsers, YouTube downloads and screen recorders that run in a web page. Many phones, Apple apps, video editors and upload forms still want MP4. This converter re-encodes WebM's VP8 or VP9 video to H.264 and its Opus or Vorbis sound to AAC, giving an MP4 that plays everywhere. It runs in your browser with ffmpeg (about 30 MB, downloaded once); files are not uploaded.
How to use this tool
- Choose one or more .webm files.
- Pick a quality; CRF 22 looks like the original for most clips.
- Optionally cap the resolution or fix the frame rate.
- Press Calculate and download the MP4.
Formula
VP8/VP9 → H.264 (x264, CRF) · Opus/Vorbis → AAC 192 kbps
MP4 cannot reliably carry VP8 or Opus, so both are converted. Odd frame sizes are rounded to even numbers, which H.264 requires. Browser screen recordings have a variable frame rate and no frame-rate field (they report 1000 fps); automatic mode turns them into steady 30 fps video so editors and phones handle them.
Example
A 5-minute 1080p screen recording of 180 MB from a browser's recorder becomes an MP4 of about 40–90 MB at CRF 22 — static screen content compresses very well in H.264. Encoding takes roughly 15–30 minutes in a browser tab on a laptop; choosing 720p makes it about a third faster.
Frequently asked questions
Why does it take longer than a desktop converter app?
Desktop apps use every processor core and hardware encoders. A web page runs the encoder on one core, so expect about 12 frames per second at 720p and 6 at 1080p on a recent computer. In return, nothing is installed or uploaded.
What happens with AV1 WebM files?
The in-browser engine cannot decode AV1, so the AV1 video is copied into the MP4 unchanged and only the sound is converted. Such MP4s play in current browsers and recent phones but not on older devices.
Is transparency kept?
No. MP4 with H.264 has no alpha channel; transparent areas become black.
Are my files uploaded?
No, everything happens in your browser.
Last updated:
Page address with these inputs: https://kakao.mobile/tools/webm-to-mp4-tool/
Cite or embed this tool
kakaoCalc. (2026, November 8). WebM to MP4 Converter. https://kakao.mobile/tools/webm-to-mp4-tool/
“WebM to MP4 Converter.” kakaoCalc, 8 Nov. 2026, kakao.mobile/tools/webm-to-mp4-tool/.