Aspect Ratio Calculator.
Give it a width and height and it reduces them to a clean ratio like 16:9. Then resize without distortion: type a new width and get the height that keeps the same proportions (or the other way round). Everything runs in your browser.
Ratios, reduced.
An aspect ratio is just width ÷ height, written as two whole numbers with no common factor. To reduce 1920 × 1080, divide both by their greatest common divisor (120): you get 16 : 9. The same method turns 1280 × 720 into 16 : 9 too — which is why they look identical on screen, only at different pixel counts.
Keeping the ratio when resizing is the whole game for avoiding stretched images and letterboxing: if the original is w × h, a new width W needs height W × h ÷ w. This tool does that rounding to whole pixels, so the result may be off by a fraction of a pixel from a perfect ratio — unavoidable when pixels are integers.
Common ratios worth recognizing: 16:9 (video, most monitors), 4:3 (older displays), 1:1 (square social posts), 21:9 (ultrawide), 3:2 (many cameras). Nothing here is uploaded.