How to use the WEBM to MP4 converter
- Select the WebM recordings.
- Choose Balanced for most captures, or Small file when the clip is only going into a chat.
- Convert and download the MP4s.
Why this is a true encode
Inside a WebM you will find VP8, VP9 or AV1 picture with Opus or Vorbis sound, none of which older MP4 players expect. So both streams are rebuilt, video in H.264 and audio in AAC, and the time it takes depends on resolution and length rather than just file size.
Screen captures with flat backgrounds and text often come out the same size or smaller, since H.264 deals well with that kind of content. Camera footage in VP9 may grow a little, VP9 being the more efficient format.
How fast it runs
When WebCodecs in your browser offers encoders for H.264 and AAC alike (Chrome on Windows or macOS does), the hardware encoder takes over and finishes faster than the clip plays. Where AAC is missing, Chrome on Linux for instance, ffmpeg in WebAssembly handles the file on a single CPU core.
On that slower path a typical laptop manages roughly 20–40 frames a second at 720p, so a ten-minute capture needs a few minutes.
Fixing recordings that will not seek
Recorders built on the browser’s MediaRecorder often write WebM files with no duration and no seek index. The converted MP4 carries a proper index, so it shows its length and jumps to any point straight away.
Variable frame rate from screen capture is kept as recorded, which keeps voice and picture in sync.
Questions
Can I just rename the file to .mp4?
The contents would not change. Apple devices, PowerPoint and most social sites inspect the codecs and still reject it.
What happens to a transparent WebM?
MP4 with H.264 cannot store transparency, so those areas come out black.
Can I shrink a 1080p capture to 720p?
Yes. Choose 720p under More settings; videos already smaller than the chosen height are never enlarged.