How this tool works
The image is drawn onto a canvas at exactly the width and height you enter. Both are used as given, which means the proportions are not protected: enter numbers that do not match the original ratio and the image stretches or squashes to fit them. That is worth knowing before you use it, because the tool will do exactly as asked without warning that the result is distorted.
The output is a PNG, so the result is lossless and any transparency survives. Scaling is done by the browser's own image smoothing, which is a good general-purpose resampler — better than nearest-neighbour, not as good as a dedicated tool at very large reductions.
Getting a resize that looks right
- Work out the second dimension first: divide the original height by the original width and multiply by your new width. The Aspect Ratio Calculator in this category does exactly that arithmetic.
- Reducing is safe, enlarging is not: making an image smaller discards information cleanly. Making it larger invents nothing — the result is the same detail spread over more pixels, and it looks soft.
- Large reductions in one step lose detail: shrinking to a small fraction of the original in a single pass can leave fine detail muddy. Two moderate steps often look better than one drastic one.
- PNG output is lossless and large: a resized photograph will be much bigger than the JPEG it came from despite having fewer pixels. Convert afterwards if the file size matters.