Ratio Calculator
Solve a proportion A : B = C : ? and simplify a ratio to its lowest terms.
About this tool
Enter the two parts of a known ratio, A and B, and the first part of the new ratio, C. The calculator cross-multiplies to find the missing value, simplifies A : B to whole numbers in lowest terms and shows it as a 1 : n unit ratio — handy for scaling recipes, mixing ratios, maps and models.
Frequently asked questions
Cross-multiply. For 2 : 3 = 10 : x, 2 × x = 3 × 10, so x = 30 ÷ 2 = 15.
Divide both parts by their greatest common divisor. 12 : 18 has a GCD of 6, so it simplifies to 2 : 3. For decimals, first multiply both parts by 10 until they are whole numbers: 1.5 : 2.5 becomes 15 : 25, then 3 : 5.
If 2 cups of flour need 3 eggs, set up 2 : 3 = 5 : x for 5 cups of flour: x = 3 × 5 ÷ 2 = 7.5 eggs.
A ratio written as 1 : n, found by dividing both parts by the first. 2 : 3 is 1 : 1.5, which says there are 1.5 parts of B for every part of A.
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