What is an aspect ratio?
An aspect ratio describes the proportional relationship between the width and height of a rectangle. It is written as two numbers separated by a colon, such as 16:9 or 4:3. Aspect ratios are widely used for screens, photos, videos, posters, and image sizing because they describe shape independently of actual size — a 1280×720 image and a 1920×1080 image both share the same 16:9 ratio.
How to use this calculator
Enter the width and the height of your rectangle in any consistent unit (pixels, centimetres, inches — it does not matter as long as both use the same unit). The calculator divides both numbers by their greatest common divisor (GCD) to reduce them to the simplest whole-number ratio, and also shows the decimal ratio (width ÷ height).
The formula explained
To simplify a ratio you find the greatest common divisor of the width and height, then divide each by it.
$$\text{Ratio} = \frac{\text{Width (px)}}{\gcd} : \frac{\text{Height (px)}}{\gcd} \qquad \text{where}\quad \gcd = \gcd\!\left(\text{Width},\, \text{Height}\right)$$For example, with 1920 and 1080 the GCD is 120. Dividing gives \(1920 \div 120 = 16\) and \(1080 \div 120 = 9\), so the ratio is 16:9. The decimal ratio is simply \(1920 \div 1080 \approx 1.7778\).
Worked example
Suppose you have an image that is 800 pixels wide and 600 pixels tall. The GCD of 800 and 600 is 200. So \(800 \div 200 = 4\) and \(600 \div 200 = 3\), giving an aspect ratio of 4:3. The decimal ratio is \(800 \div 600 \approx 1.3333\).
FAQ
Does the unit matter? No. As long as both values use the same unit, the ratio is identical.
Why are my numbers not reduced fully? Non-integer or unusual dimensions are rounded to the nearest whole number before reducing, so very precise decimals may give an approximate ratio.
What is 16:9? It is the standard widescreen ratio used for most modern TVs, monitors, and HD/4K video.
