An aspect ratio describes the shape of a rectangle (its width relative to its height) without saying anything about its size. A 1920 × 1080 video and a 3840 × 2160 one both have the shape 16:9; a 4000 × 3000 photo is 4:3 and a 6000 × 4000 one 3:2. Writing the ratio in lowest terms comes from dividing both numbers by their greatest common divisor, and the decimal form (width ÷ height, 1.78 for 16:9) is handy for comparing shapes.
Keeping the aspect ratio when you resize stops pictures from looking stretched or squashed. If you scale the width by some factor, the height must be scaled by the same factor, which is exactly what the resize modes do. You can start from a pixel size, or from a ratio such as 21:9 and one side you already know.
Not every resolution reduces to a familiar ratio: 1366 × 768 is really 683:384 (about 16:9), and 2560 × 1080 “ultrawide” is 64:27, marketed as 21:9. The calculator therefore also names the closest common format and how far your shape is from it.
How to use the Aspect Ratio Calculator
- I want to
- Original width
- Original height
- Ratio width
- Ratio height
- New width: Leave 0 if you only need the ratio.
- New height
Everything is calculated in your browser, so results appear instantly and nothing you type is sent to a server.
How it works
For a pixel size, the width and height are divided by their greatest common divisor to get the ratio. When resizing, the missing side is the known side multiplied by the ratio of the original height to width (or the other way round).
Worked example
A 1920 × 1080 video: gcd(1920, 1080) = 120, so the ratio is 16:9 (1.7778). To resize it to 1280 px wide, the height is 1280 × 1080 ÷ 1920 = 720 px. With a 21:9 ratio and a width of 3440 px, the height is 3440 × 9 ÷ 21 ≈ 1474 px (real 3440 × 1440 monitors are 43:18, slightly wider).
Common aspect ratios
| Ratio | Decimal | Typical use | Example size |
|---|---|---|---|
| 1:1 | 1.00 | Social posts, square prints | 1080 × 1080 |
| 4:3 | 1.33 | Phones, Micro Four Thirds, old TV | 4000 × 3000 |
| 3:2 | 1.50 | DSLR and mirrorless photos | 6000 × 4000 |
| 16:10 | 1.60 | Laptops, monitors | 1920 × 1200 |
| 16:9 | 1.78 | HD and 4K video, TVs | 3840 × 2160 |
| 21:9 | 2.33–2.39 | Ultrawide monitors, cinema | 2560 × 1080 |
Formula
Ratio = (width ÷ gcd) : (height ÷ gcd) · New height = new width × height ÷ width · New width = new height × ratio width ÷ ratio height
Worked example
Inputs: I want to: Find the ratio of a size and resize it; Original width: 1,920; Original height: 1,080; New width: 1,280.
Results: Aspect ratio = 16:9; New height = 720; Ratio width = 16; Ratio height = 9; Decimal ratio (width ÷ height) = 1.7778; Closest standard format = 16:9 – exact match.
Key terms
- Aspect ratio
- Width divided by height, written as w:h.
- Greatest common divisor (GCD)
- The largest whole number that divides both sides exactly.
- Letterboxing
- Black bars added above and below to fit a wide picture in a taller frame.
Frequently asked questions
What is 1920 × 1080 as a ratio?
16:9: both numbers divide by 120.
What aspect ratio is 2560 × 1080?
Exactly 64:27 (2.37:1), sold as "21:9" ultrawide.
Is 1366 × 768 16:9?
Almost. It is 683:384, about 1.779, within 0.1% of 16:9.
How do I resize without distortion?
Multiply both sides by the same factor. For 1920 × 1080 to a width of 1280, the height is 1280 × 1080 ÷ 1920 = 720.
What is the aspect ratio of an iPhone screen?
Recent models are about 19.5:9 (2.17:1) in portrait or landscape.
What ratio do cameras shoot?
Most interchangeable-lens cameras are 3:2, Micro Four Thirds and phones are 4:3, and video is usually 16:9.