What aspect ratio actually describes
An aspect ratio is the relationship between an image's width and its height. It describes the shape of a frame, not its size. A postage stamp and a cinema screen can have the same aspect ratio. One is just much larger.
When you see 16:9, read it as "16 units wide for every 9 units tall". The units don't matter: pixels, inches or centimetres all work, as long as both sides use the same unit.
Two ways to write the same ratio
Ratios are written in two common forms:
- Whole-number form, such as
16:9,4:3or1:1. This is common in design, social media and consumer video. - Decimal form, such as
1.78:1or2.39:1. This is common in film and broadcast. You get it by dividing width by height.
| Whole-number | Decimal | Shape |
|---|---|---|
| 1:1 | 1.00:1 | Square |
| 4:5 | 0.80:1 | Portrait |
| 4:3 | 1.33:1 | Classic landscape |
| 3:2 | 1.50:1 | Photography landscape |
| 16:9 | 1.78:1 | Widescreen |
| 21:9 (approx.) | 2.33:1 | Ultra-wide |
| 9:16 | 0.5625:1 | Full-screen vertical |
How to simplify a ratio from pixel dimensions
To turn pixel dimensions into a simple ratio, divide both numbers by their greatest common divisor (GCD), the largest number that divides both evenly.
- Take the dimensions, for example
1920 × 1080. - Find the GCD of 1920 and 1080. It is
120. - Divide both:
1920 ÷ 120 = 16and1080 ÷ 120 = 9. - The simplified ratio is
16:9.
Some real-world sizes don't simplify neatly. A 1366 × 768 laptop screen simplifies to 683:384. In practice everyone calls it "16:9" because its decimal (1.7786) is almost identical. Ratio Finder Pro shows both: the familiar name and the exact ratio.
The three formulas you will use
ratio = width ÷ heightheight = width ÷ ratiowidth = height × ratioExample: you need a 16:9 banner that is 1500 px wide. Height = 1500 ÷ 1.7778 ≈ 844 px. For an exact answer, use the fraction instead: 1500 × 9 ÷ 16 = 843.75. Round to 844, because pixels must be whole numbers.
Orientation: landscape, portrait and square
- Landscape: width is greater than height (ratio above 1), for example 16:9.
- Portrait: height is greater than width (ratio below 1), for example 4:5 or 9:16.
- Square: width equals height (ratio exactly 1).
Swapping the two numbers keeps the same proportions but rotates the orientation: 16:9 becomes 9:16.
Scaling without stretching
An image keeps its aspect ratio when both sides are multiplied by the same number. Halving 1920 × 1080 gives 960 × 540, still 16:9. If you change only one side, the image stretches or squashes, and faces and circles look wrong.
Rule of thumb: resize to change the size, crop to change the shape. Never type in a new width and height independently unless you mean to change the shape.
Most editors have a "constrain proportions" or lock icon for this. In the Ratio Finder Pro calculator, turn on Lock Ratio and editing one side updates the other automatically.
Frequently asked questions
Is aspect ratio written width first or height first?
Width first, then height. 16:9 is a wide landscape shape and 9:16 is a tall portrait shape.
Why is 21:9 not exactly 2.39:1?
"21:9" is a marketing name. True 21:9 is 2.33:1, while cinema scope is about 2.39:1, and many ultra-wide monitors are 2.37:1 (3440 × 1440). They are close enough to share the label.
Can an aspect ratio have decimals?
Yes. Decimal ratios such as 1.85:1 and 2.39:1 are standard in film. Whole-number and decimal forms describe the same thing.