Rule of Three Calculator
Fill in three values and the fourth is calculated instantly, with the detailed working. Perfect for a recipe, a price, a map scale or a maths exercise.
Your values
Type of proportion
Fill in three boxes and leave empty the one to find.
Quantity 1Quantity 2
Result
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About the rule of three
The rule of three finds an unknown value when two quantities are proportional: if 3 kg cost 4.50, how much do 7 kg cost? Multiply across (4.50 × 7), then divide by the third value (3): 10.50.
Features
- Any box: leave any of the four boxes empty and it is calculated
- Detailed working: the formula with your numbers and the ratio
- Inverse proportion: for situations where one quantity goes down when the other goes up
- Comma or dot: both are accepted as decimal separator
How to use
- Step 1: on the first row, enter two matching values (3 kg → 4.50)
- Step 2: on the second row, enter the known value (7 kg)
- Step 3: the empty box shows the result (10.50)
Examples
- Cooking: a recipe for 4 people uses 250 g of flour; how much for 6? (375 g)
- Price: 12 for 5 m of fabric; how much for 8 m? (19.20)
- Map: 1 cm for 25 km; how many km for 7.4 cm? (185 km)
- Inverse proportion: 3 workers take 12 days; how long for 4 workers? (9 days)
Tips
- Keep the same units in a column (kg with kg, price with price)
- Make sure the situation really is proportional: age and height, for example, are not
Privacy
All calculations run in your browser: the values you enter are never sent anywhere and we never have access to them.