What Is the Rule of Three?
The rule of three is a mathematical method used to solve proportional problems when three values are known and one value is unknown.
How the Rule of Three Works
When two values are proportional to two other values, the missing value can be found using multiplication and division.
The Formula Behind the Calculation
The missing value is calculated by multiplying the corresponding values and dividing by the remaining known value.
Practical Shopping Example
If 2 kilograms of apples cost $6, then 5 kilograms cost $15. The rule of three makes this calculation quick and easy.
Using the Rule of Three for Percentages
Percentage calculations are among the most common applications of proportional reasoning.
Using the Rule of Three in Recipes
Recipes often require ingredient quantities to be scaled up or down while maintaining the same proportions.
Business and Financial Applications
Businesses use proportional calculations for pricing, profit estimates, discounts, commissions and forecasting.
Using This Rule of Three Calculator
Enter the three known values and the calculator instantly determines the missing proportional value.