WebM to MP4

Why WebM needs converting

Screen recorders, browser extensions, Loom-style apps and many downloads save video as WebM, Google's open format with VP8, VP9 or AV1 video. It plays in browsers, but iPhones, some Macs and TVs, PowerPoint, WhatsApp on older phones and many editors want MP4 with H.264, the most widely supported video codec.

Unlike changing the container of an MP4 or MOV, WebM's video has to be re-encoded: each frame is decoded and compressed again as H.264. That takes some time, roughly the length of the video or less on a recent computer, and the progress bar shows how far along it is. Sound is copied when MP4 can hold it as is, and re-encoded otherwise.

You can convert up to five videos in a row with one click.

Done on your device

The re-encoding uses your browser's built-in video encoder, so the video never leaves your device and there is no size or daily limit.

In a Web Worker, Mediabunny reads the WebM in slices and feeds the frames to your browser's built-in video decoder and H.264 encoder (WebCodecs), at about 4 Mbps, which keeps screen text and motion sharp. Audio MP4 can hold (AAC, MP3, Opus) is copied; Vorbis is re-encoded. The MP4 is written to the browser's private storage as it is produced, then offered for download.