Why WebM causes trouble
Browsers record in WebM because it is royalty-free, but iPhones, most TVs, Premiere and DaVinci Resolve will not open it. Converting to H.264 MP4 fixes that at the cost of one re-encode.
What conversion actually does
The VP8/VP9 video is decoded and re-encoded as H.264, then written into an MP4 container. Frame timing comes from the source, so duration and frame rate are preserved exactly.
Why WebM will not play on an iPhone
WebM is a free, open format that browsers support well and consumer software largely does not. Safari has never supported it fully, iOS will not play it in most contexts, and plenty of editing software and televisions reject it outright.
MP4 with H.264 is the opposite: patent-encumbered, and playable on essentially every device made in the past fifteen years. Which is why the conversion is nearly always in this direction.
Frequently asked questions
Is my file uploaded?
No. Everything is parsed and processed inside your browser — the file never leaves your device.
Why convert WebM to MP4 at all?
Compatibility. WebM works in browsers and little else; H.264 MP4 plays on essentially every device and in every editor.
Does converting lose quality?
A little, once — both are lossy, so the video is re-encoded. At a sensible bitrate the difference is not visible at normal viewing size.
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