Formula
Both sides are divided by their greatest common divisor:
ratio = width / gcd : height / gcd
1920 and 1080 share a divisor of 120, which leaves 16:9. The page does not crop an image. It only names the shape.
Assumptions
- Width and height are whole counts, usually pixels.
- The ratio uses the full frame, including any bars you have already counted.
- One ratio is published for screens, photos, and print.
Worked example
1920 by 1080 reduces to 16:9.
Input definitions
- Width
- The horizontal count.
- Height
- The vertical count.
Limitations
A ratio does not say whether the image is sharp or whether it fits a paper size. Use print size for paper and PPI for a screen.
Questions
Why is 1920 by 1080 not left as 1920:1080?
Both numbers divide by 120. The reduced form 16:9 is the same shape.
Can I enter inches instead of pixels?
Yes, if both sides use the same unit and you enter whole numbers. A 16 by 9 print is also 16:9.