How compressing to a size works

Most compressors ask for a quality and leave you to find out how big the result is. FitSize starts from the size instead. It works out how many bits the whole file can spend, sets aside what the audio needs, and gives what is left to the picture. If that is too little for your resolution, it steps the resolution down a rung until each frame gets enough bits to look reasonable.

It never aims exactly at your target. A small margin is kept back for the container and for the encoder running a little over, so the file lands under the number, not on it.

Why it runs in your browser

The video is read, compressed and saved by your own browser, using the hardware video encoder when it has one and a slower software fallback when it does not. Nothing is uploaded to a server, and you can confirm that in your browser's network panel. The compressing code is only downloaded after you choose a file, so this page loads quickly.

Which size to pick

If you know where the file is going, use that destination's page: it aims for the size the platform will really accept, which for email is smaller than the number on the box. If you only know a number, pick it from the sizes below.