Rule of Three Calculator: Simple, Compound and Inverse
The rule of three finds the fourth value of a proportion when three are known. If 40 liters cover 480 km, 25 liters cover 25 × 480 ÷ 40 = 300 km. Here you solve simple or compound problems, mark each quantity as direct or inverse and see the calculation step by step.
Your numbers stay in this tab
Which rule of three
Case 1Case 2Quantity B: the value you are looking for
x always goes in case 2. If the problem asks about the first case, swap the two rows.
How the two quantities vary
If unsure, double one quantity in your head: if the other doubles, it is direct; if it halves, it is inverse.
Use an example:
For discounts, increases and percentage change, the calculation is ready in the Percentage Calculator
Result
Fill in the three known values to see x.