What is an aspect ratio?
An aspect ratio describes the proportional relationship between the width and height of an image or video. It is written as two numbers separated by a colon, such as 16:9 or 4:3. The same shape can be displayed at many resolutions — 1920×1080, 1280×720 and 3840×2160 all share the same 16:9 aspect ratio. This calculator converts any pixel resolution into its simplest aspect ratio so you can quickly identify the format you are working with.
How to use this calculator
Enter the width and height of your image or video in pixels, then read the simplified ratio. The tool also shows the decimal ratio (width divided by height) and the greatest common divisor used to reduce the fraction. This is handy when configuring cameras, exporting video, designing thumbnails, or matching a display resolution.
The formula explained
To reduce a resolution to its lowest-terms ratio, divide both the width and the height by their greatest common divisor (GCD). The GCD is the largest whole number that divides both values exactly. Computing width ÷ GCD and height ÷ GCD gives the smallest pair of integers that preserve the original proportions.
$$\text{Ratio} = \frac{\text{Width}}{\gcd} : \frac{\text{Height}}{\gcd} \qquad \text{where}\quad \gcd = \gcd\!\left(\text{Width},\ \text{Height}\right)$$
Worked example
Take a Full HD resolution of 1920×1080. The greatest common divisor of 1920 and 1080 is 120. Dividing each side gives \(1920 \div 120 = 16\) and \(1080 \div 120 = 9\), so the aspect ratio is 16:9. The decimal ratio is \(1920 \div 1080 \approx 1.7778\).
FAQ
Why is my ratio not a "nice" number like 16:9? Some resolutions do not reduce to common ratios. For example 1366×768 reduces to 683:384, which is close to but not exactly 16:9.
Does this work for photos as well as video? Yes. The calculation is purely geometric, so it applies to any image, screen, or video frame measured in pixels.
What is a decimal ratio useful for? A single decimal makes it easy to compare formats — \(16:9 \approx 1.78\) is wider than \(4:3 \approx 1.33\).
