Solve A / B = C / D
Enter three values. The blank field is the value the calculator will find.
What is a proportion?
A proportion is an equation that says two ratios are equal. It is often written as two fractions, such as A/B = C/D. For example, 2/5 = 8/20 because both fractions represent the same relationship.
How to solve a proportion
Enter the two values in one fraction and one known value in the other. Then leave the unknown value blank and select Solve proportion. The calculator uses cross multiplication to find the missing value.
Worked example
Suppose 2/5 = X/20. Cross multiplication gives 2 × 20 = 5 × X. Therefore, X = 40 ÷ 5 = 8. The completed proportion is 2/5 = 8/20.
Where proportions are used
Proportions are useful for scaling recipes, converting measurements, working out prices by quantity, map scales and comparing rates. For example, if 8 hours of work earns £120, then 25 hours earns £375 when the rate stays the same.
Direct and inverse proportion
In a direct proportion, both quantities move in the same direction: if one doubles, the other doubles. In an inverse proportion, one quantity increases while the other decreases, such as more workers completing the same task in fewer hours. This calculator solves direct proportional relationships written as equal ratios.