> calculator rule-of-three
Rule of three calculator
Solve a rule of three in seconds: you know three numbers, the calculator finds the fourth and shows the steps. It works for prices, quantities, recipes, distances and anything else that grows in the same proportion.
Example: 3 bread rolls cost €4.50. What do 7 cost?
Related: Percentage calculator · Unit converter · Scale a recipe to other servings (German) · VAT
The calculation runs only on your device.
How it works
- 01The top line holds what you know: quantity and value, for example 3 bread rolls and €4.50.
- 02The bottom line holds the new quantity, for example 7 bread rolls.
- 03The result appears at once, the steps below it.
Rule of three: the formula. First scale down to one, then scale up to the new quantity: value ÷ quantity × new quantity. These are the three steps that give the rule of three its name and lead you to the result.
Example 1: price. 3 bread rolls cost €4.50. One roll costs 4.50 ÷ 3 = €1.50, seven cost 1.50 × 7 = €10.50.
Example 2: fuel. Your car needs 6.5 liters per 100 km. For 350 km that is 6.5 ÷ 100 × 350 = 22.75 liters.
Example 3: recipe. A dough for 4 people needs 500 g of flour. For 6 people it is 500 ÷ 4 × 6 = 750 g.
The calculator solves the direct rule of three: more quantity, more value. It does not calculate the inverse rule of three (more people, less time). There the rule is value × quantity ÷ new quantity: if 3 people need 8 hours, 4 people need 8 × 3 ÷ 4 = 6 hours.