How to use the MP4 to WEBM converter
- Add the MP4 or M4V clips.
- Pick High, Balanced or Small file, and lower the resolution under More settings if the clip is going onto a web page.
- Convert to WebM and save the files one at a time or as a ZIP.
Two encoding routes
On the fast route, WebCodecs decodes the H.264 picture and encodes VP9, or VP8 if VP9 encoding is not available, often quicker than the clip plays. The soundtrack becomes Opus; if the MP4 already carries Opus it is copied rather than re-encoded.
When a browser lacks WebCodecs, the job moves to ffmpeg compiled to WebAssembly. That single-thread engine is fetched once from jsDelivr and writes VP8 video, which every WebM-capable browser can play. It is slower, but the result is the same kind of file.
Header loops and page videos
A looping header video rarely needs full HD or sound. Choose 720p, keep the clip short, and run it through the mute tool afterwards so the page carries no audio track it will never play.
Put the WebM and the original MP4 side by side as two sources in one video element. Browsers pick the first they support, and older iPhones and TVs fall back to the MP4.
What the quality choice changes
High keeps texture and film grain, Balanced suits ordinary footage, and Small file is meant for previews. Each step trades detail for bytes; dropping from 1080p to 720p usually saves more than moving down a quality step.
At the default setting a WebM is often about the size of the MP4 it came from, because the encoder aims for a clean picture. When all you want is a lighter clip that stays MP4, use the video compressor instead.
Questions
Will a transparent WebM come out of this?
No. H.264 in an MP4 has no alpha channel, so there is nothing transparent to carry across.
Does Safari play the result?
Current Safari versions play VP9 WebM. Keep the MP4 as a fallback for older Apple devices.