Aspect ratio is the proportional relationship between an image or screen's width and its height. It's expressed as two numbers separated by a colon — 16:9, 4:3, or 1:1 — where the first number represents width and the second represents height.
A 1920×1080 pixel image and a 1280×720 pixel image both have the same 16:9 aspect ratio, even though their pixel counts are very different. The ratio describes the shape, not the size.
How to calculate aspect ratio
The calculation uses the greatest common divisor (GCD) to simplify the ratio to its smallest whole-number form.
For a 1920×1080 image:
- GCD(1920, 1080) = 120
- 1920 ÷ 120 = 16
- 1080 ÷ 120 = 9
- Aspect ratio: 16:9
For a 800×600 image:
- GCD(800, 600) = 200
- 800 ÷ 200 = 4
- 600 ÷ 200 = 3
- Aspect ratio: 4:3
Common aspect ratios and their uses
| Ratio | Name | Common uses |
|---|---|---|
| 16:9 | Widescreen | YouTube, HD monitors, desktop, TV |
| 4:3 | Standard | Old TVs, tablets, presentations |
| 1:1 | Square | Instagram post, profile pictures |
| 9:16 | Portrait | TikTok, Instagram Stories, mobile video |
| 21:9 | Ultrawide | Cinema, ultrawide monitors |
| 4:5 | Portrait post | Instagram portrait |
| 3:2 | Photo | DSLR cameras, 35mm film |
| 2:3 | Portrait photo | Portrait photography |
| 5:4 | Near square | Older computer monitors |
Standard resolutions at 16:9
| Name | Width | Height |
|---|---|---|
| 720p (HD) | 1280 | 720 |
| 1080p (Full HD) | 1920 | 1080 |
| 1440p (QHD) | 2560 | 1440 |
| 4K (UHD) | 3840 | 2160 |
Resizing while keeping the aspect ratio
When you resize an image, maintaining the aspect ratio prevents stretching or squishing. The formula is:
New height = New width ÷ (Original width ÷ Original height)
Example: Resize a 1920×1080 image to 1280 wide:
- Ratio = 1920 ÷ 1080 = 1.7778
- New height = 1280 ÷ 1.7778 = 720
- Result: 1280×720 (still 16:9)
Fitting within a bounding box
A common problem: you have an image of unknown dimensions and need it to fit inside a container with a maximum width and height, without cropping.
For a 1920×1080 source image that must fit inside 800×400:
- Try fitting to width: 800 wide → 800 ÷ 1.7778 = 450 high → too tall (450 > 400)
- Try fitting to height: 400 high → 400 × 1.7778 = 711 wide → fits (711 < 800)
- Result: 711 × 400
Aspect ratio vs. resolution
These are related but different:
- Aspect ratio describes the shape (16:9)
- Resolution describes the number of pixels (1920×1080)
Two images with the same aspect ratio can have completely different resolutions. Higher resolution means more pixels, which means more detail and larger file size — but the shape is the same.
How to calculate aspect ratios for free
- Go to Aspect Ratio Calculator
- Ratio from dimensions: Enter width and height to get the simplified ratio and nearest standard ratio
- Resize keeping ratio: Enter original dimensions, type a new width or height, get the other dimension instantly
- Dimensions from ratio: Enter a ratio like "16:9" and a base dimension to calculate the other
- Use the quick resolution buttons (720p, 1080p, 1440p, 4K) for standard sizes
- Click any ratio in the reference table to use it as an example