With proportions kept, fill in one side and the other is worked out for you; fill in both and the picture is fitted inside those two numbers, so one side usually comes out smaller than the number you typed. Turn proportions off and both numbers are used exactly, with the stretching that comes with them.
This page loads no libraries at all. The work is done by the browser itself. Your file is read on your own machine and never sent anywhere. Open your browser’s network tab and watch while you use it.
With proportions kept, the picture is fitted inside them: it meets one of your numbers and stays under the other, and nothing is cropped to make it fit. Turn proportions off and both numbers are used exactly, which stretches the picture. Leave both at 0 and it stops and asks you for one.
A PNG comes back as a PNG, with its transparency intact. Everything else comes back as a JPEG at 92% quality, WEBP included, so a transparent WEBP has those areas filled with white. An animated GIF comes back as a single still frame.
For anything that is not a PNG, yes: it is decoded, drawn at the new size and encoded again at 92% quality. A PNG is redrawn as well, but PNG encoding loses nothing. Either way, work from the original rather than resizing a file you have already resized.
You can. Nothing here stops you, and the scaling is smoothed, so it comes out soft rather than blocky. There is no detail to recover though. The added pixels are worked out from the ones beside them.
Everything here is free and works the same way, in your browser.
A new DeskKit tool lands every week or two. Leave an address to hear about it. Nothing else is ever sent, and one click removes you.