Ratio and Proportion Calculator
Solve ratios and proportions (a:b = c:d) — find the unknown value from the three known ones.
Tip: If A:B = C:D, then A × D = B × C (the cross product).
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About this digital tool
The ratio and proportion calculator solves the equation A : B = C : D when one of the four values is missing. Choose which value you are looking for (A, B, C or D), enter the three known ones and the calculator finds the unknown, also showing a check of the result.
The rule is based on cross-multiplication: A × D = B × C. For example, if 2 : 5 = 6 : D, then D = 5 × 6 ÷ 2 = 15. Check: 2 × 15 = 30 and 5 × 6 = 30. This is used for map scales, for recipes that need scaling, for mixtures (e.g. paint or concrete) and for any task where two quantities grow in the same ratio.
How to use
- Enter the three known values in the proportion A:B = C:D.
- Leave blank the field of the value you want.
- See the calculated result.
Frequently asked questions
How is a proportion solved?
If A:B = C:D and D is unknown: D = (B × C) ÷ A. The same for any other value — use the cross product.
Where is ratio used?
Cooking recipes (scaling quantities), maps (scale 1:50,000), concrete mixes (1:2:3), pharmacology, and any comparison of two quantities.
How is a ratio simplified?
Divide both terms by their greatest common divisor. E.g. 12 : 18 divided by 6 becomes 2 : 3.
How do I check whether a proportion is correct?
Cross-multiply: if A × D equals B × C, the proportion holds. E.g. 2 : 5 = 6 : 15 is correct because 2 × 15 = 5 × 6 = 30.