An aspect ratio expresses a shape's width relative to its height, independent of size. Enter any width and height and the tool divides both by their greatest common divisor (found with the classic Euclidean algorithm) to give the simplest whole-number ratio — 1920×1080 reduces to 16:9, 1024×768 to 4:3, and 2560×1080 to 64:27. Common video and screen ratios such as 16:9, 4:3, 21:9 and 1:1 all fall out of this single reduction step, so you never have to memorise them.
Tick "lock this ratio" to keep the proportions fixed, then type a new width and the matching height is computed as width × (ratioH / ratioW) — or type a height to get the width. This is exactly what you need when resizing a video frame, image or layout without stretching it. Non-integer results are rounded for display. Everything runs locally in your browser and nothing is uploaded.