Ratio Calculator
Solve any proportion A:B = C:D — leave one value blank to find it by cross-multiplication.
Ratio A : B = C : D (leave one blank to solve)
Solved for D
12
Equation
3:4 = 9:12
Ratio as fraction
3 ÷ 4
How to Use This Calculator
Enter three of the four values in the proportion A:B = C:D and leave one blank to solve for it. The calculator uses cross-multiplication to find the missing value. You can also fill all four values to check whether two ratios are equal.
The Formula
Proportion: A/B = C/D
Cross multiplication:
Solve D: D = (B × C) ÷ A
Solve C: C = (A × D) ÷ B
Solve B: B = (A × D) ÷ C
Solve A: A = (B × C) ÷ D
Examples
Recipe scaling: 3 cups flour for 12 cookies — how much for 20 cookies?
A:B = C:D3:12 = ?:20
D = (12 × ?) ÷ 3, solve CC = (3 × 20) ÷ 12 = 5 cups
Currency: ₦1,600 = $1 → how much is $350?
1:1600 = 350:DD = 1600 × 350 = ₦560,000
FAQ
Cross-multiplication is the technique used to solve proportions. Given A/B = C/D, multiply diagonally: A × D = B × C. This comes from the fact that equal fractions have equal cross-products. To solve for D: D = (B × C) ÷ A. It's taught in Nigerian secondary school mathematics under "ratio and proportion."
For a 3:2 split of ₦500,000: total parts = 3+2 = 5. Each part = ₦100,000. First share = 3 × ₦100,000 = ₦300,000. Second share = 2 × ₦100,000 = ₦200,000. Using this calculator: set A=3, B=5, C=500000, solve D — but this gives a different format. Better to use: one share = 500000 × 3/5 = 300,000; other = 500000 × 2/5 = 200,000.
Proportions appear throughout Nigerian business: scaling recipes for event catering, calculating unit costs when buying in bulk, currency conversion, map reading for land surveys, diluting chemicals (e.g. for cleaning or agriculture), and calculating staff profit shares. The ratio 3:2 appears in many partnership agreements under Nigerian company law.