How to Calculate an Aspect Ratio
To calculate an aspect ratio, divide the width and the height by their greatest common divisor (GCD) — the biggest number that divides both evenly. What is left is the ratio in its simplest form: 1920×1080 ÷ 120 becomes 16:9.
The three steps
- Write the pair as a fraction. Width over height: 1920 / 1080.
- Find the GCD. The largest number dividing both 1920 and 1080 is 120. (Any common factor works if you repeat the step — dividing by 10, then 4, then 3 also gets there: 1920/1080 → 192/108 → 48/27 → 16/9.)
- Divide both sides. 1920 ÷ 120 = 16, 1080 ÷ 120 = 9. The aspect ratio is 16:9.
Worked examples
| Dimensions | GCD | Division | Aspect ratio | Decimal |
|---|---|---|---|---|
| 1920×1080 | 120 | 1920÷120, 1080÷120 | 16:9 | 1.7778:1 |
| 1080×1350 | 270 | 1080÷270, 1350÷270 | 4:5 | 0.8:1 |
| 2560×1080 | 40 | 2560÷40, 1080÷40 | 64:27 | 2.3704:1 |
| 1366×768 | 2 | 1366÷2, 768÷2 | 683:384 | 1.7786:1 |
| 3000×2000 | 1000 | 3000÷1000, 2000÷1000 | 3:2 | 1.5:1 |
The 2560×1080 row is the instructive one: it is sold as a "21:9" monitor, but the honest simplified ratio is 64:27 (21:9 would simplify further to 7:3 = 63:27). And 1366×768 shows why some ratios look ugly — the GCD is just 2, so nothing cancels. Ugly is still correct.
Scaling to a new size
Once you have the ratio (call it W:H), either side can be found from the other:
- New height = new width × H ÷ W — e.g. at 16:9, width 1280 → height 1280 × 9 ÷ 16 = 720.
- New width = new height × W ÷ H — e.g. at 4:5, height 1000 → width 1000 × 4 ÷ 5 = 800.
If the result is not a whole number, the target size cannot be expressed in whole pixels at that exact ratio — round to the nearest pixel (the error is invisible) or pick a width that is a multiple of W so the division comes out exact. The calculator on the home page does all of this live, and each ratio page (16:9, 4:3, 21:9…) starts pre-filled with a size in that ratio.
Frequently asked questions
How do I find the GCD quickly?
Use the Euclidean algorithm: divide the larger number by the smaller and keep the remainder; repeat with the smaller number and the remainder until the remainder is 0. The last non-zero remainder is the GCD. For 1920 and 1080: 1920 = 1080×1 + 840; 1080 = 840×1 + 240; 840 = 240×3 + 120; 240 = 120×2 + 0 — so the GCD is 120.
Does the order matter — is 9:16 the same as 16:9?
No. Width always comes first. 16:9 is landscape (wide), 9:16 is the same proportions rotated to portrait. Swapping the order turns the picture on its side.
Can an aspect ratio have decimals, like 1.85:1?
Yes — cinema often quotes ratios as "something : 1" (1.85:1, 2.39:1). It is the same shape described differently: 1.85:1 corresponds to the fraction 37:20. Multiply both sides of the decimal form by whatever clears the decimals to get the fraction form.
What about inches or centimetres instead of pixels?
The method is identical for any unit, as long as both sides use the same one. A 6×4 inch print is 3:2 whether you measured it in inches, centimetres (15.24×10.16) or pixels.