Aspect Ratio Calculator — width and height to ratio

How it works

An aspect ratio describes the shape of a rectangle — a screen, an image, a sheet of paper — as the proportion of its width to its height, independent of the actual size. A 1920 × 1080 monitor and a 3840 × 2160 monitor are the same shape, so they share an aspect ratio.

To find it, reduce the two dimensions to lowest terms by dividing both by their greatest common divisor — the largest number that divides them evenly, found with Euclid's algorithm:

ratio = (width ÷ g) : (height ÷ g), where g = gcd(width, height)

For 1920 and 1080, g = 120, so the ratio is 1920 ÷ 120 : 1080 ÷ 120 = 16 : 9. The same shape written as a single number is the decimal ratio, width ÷ height, which is 1920 ÷ 1080 = 1.78 — handy when a ratio doesn't reduce to small, familiar numbers.

Worked example

Take a 1920 × 1080 display. The greatest common divisor of 1920 and 1080 is 120, so the reduced ratio is 1920 ÷ 120 : 1080 ÷ 120 = 16 : 9. The decimal ratio is 1920 ÷ 1080 = 1.7778. A 4K panel at 3840 × 2160 has a GCD of 240 and reduces to the very same 16 : 9 — it is a bigger screen of the same shape.

Frequently asked questions

What is the aspect ratio of 1920 × 1080?

It is **16 : 9**. Both numbers share a greatest common divisor of 120, and 1920 ÷ 120 = 16 while 1080 ÷ 120 = 9. As a decimal that is 1920 ÷ 1080 ≈ 1.78. It is the standard widescreen ratio for HD and 4K displays.

How do I calculate an aspect ratio?

Divide the width and the height by their greatest common divisor (GCD) to reduce them to lowest terms. For example, 1280 and 1024 share a GCD of 256, so 1280 × 1024 is 5 : 4. The decimal version is simply width ÷ height.

What's the difference between 16:9 and 4:3?

16 : 9 is wider (a decimal ratio of about 1.78) and is used by modern TVs, monitors and video; 4 : 3 is more square (about 1.33) and was standard for older screens and many photos. The larger the decimal, the wider the rectangle.

What aspect ratio is "ultrawide"?

Ultrawide monitors are usually **21 : 9** — though 2560 × 1080 reduces exactly to 64 : 27 (about 2.37), which manufacturers round to the friendlier "21 : 9" label. Wider still is 32 : 9 "super ultrawide."

Should I use the ratio or the decimal?

Use the reduced ratio (16 : 9) when you want a recognizable shape, and the decimal (1.78) when you need to compare or compute — for instance, dividing the width by the decimal ratio gives the matching height: 1920 ÷ 1.7778 ≈ 1080.

Does this work for paper and images, not just screens?

Yes. Any width and height have an aspect ratio the same way. A 6 × 4 photo is 3 : 2, and an A-series paper sheet is about 1 : 1.41 (the square root of 2), which is why folding it in half keeps the same shape.