Why convert WebM to MP4 at all?
WebM is what your browser hands you when you screen-record with the MediaRecorder API. It is small and modern, but the moment you try to drop it into iMovie, Final Cut, Slack's upload field, a Google Doc, or a Keynote slide, things start to wobble. MP4 with H.264 is the format every player, every social platform, and every messaging app expects. So most browser-recorded videos take one extra step before they are usable. This tool is that step.
What this tool does, exactly
We re-encode the video stream to H.264 and the audio to AAC, wrap it in an MP4 container, and move the moov atom to the front so the file starts playing immediately when streamed (the +faststart flag). The pixel format is forced to yuv420p for maximum compatibility with hardware decoders. Output looks identical to the input at CRF 22 — visually lossless for screen recordings, but typically 30–50% smaller than the original WebM.
No upload — what does that mean?
Most online converters work by uploading your file to their server, transcoding it there, and handing you a download link. That is fine for a public meme, but a problem for a screen recording of a private dashboard, an internal tool, or a customer call. This tool runs FFmpeg inside your browser via WebAssembly. Your file never leaves your device — you can even disconnect the internet after the page loads and the conversion still works.
When this tool will be slower than a desktop converter
The browser version of FFmpeg is single-threaded and limited to about 2 GB of memory. For files under 500 MB it is comfortably fast. For longer recordings, a Mac doing the same job in Final Cut or HandBrake will be 3–5× faster. If you cross that line often, the Clipy desktop app handles the same conversion natively — no upload there either.
Recording a screen capture instead?
If you are about to record and you know you will need MP4 at the end, save yourself the round-trip: Clipy records straight to a shareable link with no watermark and a viewer page that does not require a signup. The whole point of this tool is to undo the WebM lock-in that happens when you record with anything else.