Aspect Ratio Calculator

The Aspect Ratio Calculator solves the proportion problem behind every resize. Give it a ratio and one dimension and it returns the other - enter 16:9 and a width of 1280 and it tells you the height is 720. Give it a width and a height and it works backwards, reducing the pair to its simplest integer ratio using the greatest common divisor.

Aspect ratio is the relationship between width and height, written as two numbers separated by a colon. Getting it wrong is what produces stretched faces in a video export, letterboxed bars on a thumbnail, or a hero image that crops badly on mobile. Because the arithmetic is a single division, most people do it in their head and land on a height that is off by a pixel or two - enough for a video encoder to complain or a CSS grid to shift.

The calculator covers the four jobs that come up in practice: solve for a missing side, reduce a size to its ratio, scale a size by a percentage, and fit a size inside a bounding box without distortion. Presets cover 16:9, 4:3, 1:1, 9:16, 3:2, 21:9, 1.91:1, and 2.39:1. Every calculation runs in your browser, works offline, and shows the formula it used.

Rounding is where an aspect ratio calculation goes wrong in practice. Exact ratios rarely produce whole pixels, so a 16:9 width of 1365 gives a height of 767.8125, and rounding that leaves the result slightly off ratio. For images that is invisible. For video it can matter: most H.264 and H.265 encoders require both dimensions to be even, and some require multiples of four, so a computed odd number needs nudging before it will encode. The simplification uses the greatest common divisor, which is why 1920x1080 reduces to 16:9 while 1366x768 reduces to 683:384 rather than to anything recognisable. A size that does not reduce cleanly is usually a sign the source was already cropped off ratio.