Aspect Ratio Calculator — Image & Video Dimensions

Aspect Ratio Calculator

The Aspect Ratio Calculator works in two directions. In resize mode you set a target ratio such as 16:9 together with one known side, and the missing width or height is returned; in ratio-finder mode you enter both pixel dimensions, and the tool reduces them to the simplest ratio they share. Both modes apply to images, video frames and any other rectangular layout.

The formula behind both modes

Scaling keeps the proportion between width and height constant, so the unknown side follows from cross-multiplication:

Missing height = (known width × ratio height) / ratio width
Missing width = (known height × ratio width) / ratio height

The ratio finder runs the opposite way: it divides both dimensions by their greatest common divisor until nothing further divides them. 1920 and 1080 share the divisor 120, so a Full HD frame reduces to 16:9. Because a screen cannot draw part of a pixel, every result is rounded to the nearest whole pixel.

Worked examples

A video thumbnail has to be 1280 pixels wide at 16:9: the height is (1280 × 9) / 16 = 720 pixels. In the other direction, a 4K frame of 3840 × 2160 pixels has a greatest common divisor of 240, and dividing both sides gives 16:9 — the same ratio as the thumbnail, which is why a 4K still scales onto a 1280 × 720 preview without cropping. Swapping the two ratio numbers flips the orientation: 16:9 becomes 9:16, the vertical frame used by Shorts and Reels, so a portrait cover 1080 pixels wide at 9:16 needs (1080 × 16) / 9 = 1920 pixels of height.

Common formats

Reference pairs produced by the same formula: