Ratio Calculator
Solve proportions A : B = C : D when exactly one value is missing. Enter three known values and calculate the fourth.
Proportion A : B = C : D
Enter three numbers and leave one field empty to find the missing value.
Tips: Leave exactly one of A, B, C, or D empty. Use the same units across both sides of the proportion. Press Enter in a field to calculate.
FAQ
A ratio compares two quantities. The notation A : B means the same relationship as the fraction A/B. In a proportion A : B = C : D, the cross products are equal: A×D = B×C.
From A : B = C : D, if D is unknown then D = (B×C)/A (when A is not zero). The tool applies the same cross-multiplication idea for whichever field you leave empty.
The calculator is built for the classic "three known, one unknown" proportion. If zero or more than one field is empty, the result is not shown until the inputs are unambiguous.