Most resizing happens because something else specified a number. An application form wants 200×230. A marketplace wants 1000 pixels on the long edge. A print template wants 1200. This page exists to hit those numbers exactly rather than approximately.
The aspect ratio lock
Leave it on and fill in one dimension: the other is calculated from your image so the proportions are preserved. This is what you want almost always, because it is the only way the result does not look subtly wrong.
Unlock it and fill in both, and the image is stretched to fit whatever you asked for. Use this only when something genuinely demands exact dimensions that do not match your image’s proportions — and be aware that faces are extremely unforgiving of it. A 5% horizontal stretch is invisible on a landscape and immediately obvious on a portrait.
Shrinking is safe, enlarging is not
Making an image smaller discards information, and browsers do that well — the result is sharp and clean. Making it larger cannot invent detail that was never captured. The extra pixels are interpolated from their neighbours, which produces a softer, slightly plasticky version of the original rather than a bigger sharp one.
- If you need a larger image, go back to the original source if one exists. A camera file, a vector, or an untouched download will always beat an upscale.
- Enlarging by more than about 200% rarely produces something usable.
- Screenshots enlarge particularly badly, because their sharp edges and text turn into soft mush.
- If the source is an SVG, do not upscale a PNG of it — render the SVG at the size you need instead.
Resize last, not first
If you are doing several things to an image, the order changes the result. Crop first, so you are not resizing parts you intend to throw away. Rotate before resizing if a quarter turn is involved, since it swaps width and height and will otherwise leave you with the dimensions transposed. Resize next. Compress last of all, so the encoder is working on the final pixel grid rather than on something that gets rescaled afterwards — rescaling compressed pixels enlarges the artifacts along with everything else.
Why the output is PNG
Resizing is an editing step, and PNG is lossless, so the resize itself introduces no compression damage. If you then want a smaller file, convert or compress afterwards. Doing it the other way round — compressing and then resizing — means the artifacts from the first step get baked in and scaled along with everything else.
One practical consequence: PNG files of photographs are large. If you are resizing a photo for the web, follow this with a compression step rather than uploading the PNG directly.
The resize happens in this tab using your browser’s own high-quality scaler, on your own device, with nothing uploaded at any point.