Resize for the slot it will fill
The most common waste on a web page is an image far larger than the space it occupies: a 4000px photo rendered in a 600px column downloads five times more data than it needs and is scaled down by the browser anyway. Measure the widest place the image is displayed, double it for high-density screens, and resize to that.
Ratio locked, one side blank
Leave height empty and set the width — the height follows from the aspect ratio. That is the right way to resize almost every time, because it cannot distort. Filling in both with the ratio locked fits the image inside the box you described, which is what you want for thumbnails of mixed portrait and landscape originals.
Downscaling is lossy in a useful way
Reducing dimensions genuinely throws away pixels, and there is no undoing it — work from the original, not from an already-resized copy. Repeatedly resizing the same file compounds softening, the same way repeatedly re-saving a JPEG compounds artefacts.
Resize before you compress
The order matters. Resizing first means the compressor is working on fewer pixels, so the same visual quality costs a smaller file. Compressing first and resizing second throws away the detail you paid for. Run this tool, then the image compressor.
Questions
Can I resize several images at once?+
Yes, up to the batch limit shown on the drop area. Anonymous visitors get one file at a time; a free account raises that to forty, which is the difference between resizing a hero image and resizing a gallery.
What happens if I set both width and height?+
With the ratio locked, the two values are treated as a box and the image is fitted inside it — so a 1600×900 image asked for 800×800 comes out 800×450, not squashed. Unlock the ratio if you genuinely want to distort the image to exact dimensions.
Should I enlarge an image here?+
You can, but there is no detail to recover — the browser interpolates, and the result is a softer version of the original at a larger size. For upscaling that adds plausible detail you need a model-based tool; this is honest bilinear scaling.
Why is my resized PNG bigger than expected?+
A photograph saved as PNG stays large regardless of dimensions, because PNG is lossless and photographs have no repeating structure to exploit. Resizing reduces the pixel count but the format is still the wrong one — convert it to WebP or JPEG as well.