How to use the ratio calculator
- A : B = C : ? — fill in three of the four boxes to find the missing value of an equivalent ratio, for example when scaling a recipe, a map or an image.
- Simplify — type a ratio such as 1920:1080 or 12:18:24 to reduce it to its simplest whole-number form.
- Divide by ratio — split an amount into parts, for example sharing 1,000 in the ratio 2:3:5.
Working with ratios
A ratio compares quantities: 3:4 means for every 3 of the first there are 4 of the second. Two ratios are equivalent when their cross-products are equal, which is how the missing value is found: if 3:4 = 15:D, then 3 × D = 4 × 15, so D = 20. Simplifying divides every part by their greatest common divisor, so 1920:1080 becomes 16:9, the standard widescreen aspect ratio. Decimals are scaled to whole numbers first, so 1.5:2.5 simplifies to 3:5.
To divide an amount in a ratio, add the parts to find the total number of shares, then give each part its fraction of the total: with 2:3:5 there are 10 shares, so 1,000 splits into 200, 300 and 500. Ratios are used in cooking, mixing concrete and paint, scale models and maps, screen and photo aspect ratios, finance (debt-to-income, price-to-earnings) and sharing costs. Calculations happen in your browser.
How do I simplify a ratio?
Divide all parts by their greatest common divisor. 12:18 ÷ 6 = 2:3.
How do I find a missing number in a ratio?
Cross-multiply and divide: for A:B = C:D, D = B × C ÷ A.
Can a ratio have more than two parts?
Yes. Simplify and divide modes accept ratios like 1:2:3:4.