Aspect Ratio Calculator handles the common ratio questions. Give it two values out of width, height, and ratio, and it finds the third.
It also reduces a pixel size to lowest terms, names the closest standard ratio, and fits a ratio inside a bounding box. It runs in your browser.
By dividing width and height by their greatest common divisor, so 1920 by 1080 becomes 16:9.
Square, 4:3, 3:2, 16:10, 16:9, the two cinema ratios, 9:16 for vertical video, and 21:9 ultrawide.
It gives the largest size with your ratio that still fits inside the width and height you set.
No. The tool runs entirely on this page.