About this tool
The rule of three is one of the most useful calculation methods in everyday life: from three known values, a fourth is calculated. If 3 kg of apples cost 7.50, how much do 5 kg cost? The method goes from the known pair to a single unit and from there to the quantity you are looking for.
The calculator solves both variants: direct proportion (“more → more”, e.g. quantity and price) and inverse proportion (“more → less”, e.g. number of workers and days needed). It shows the complete calculation path with your units – handy for homework, recipes, shopping or estimates. For percentages, use the Percentage & VAT Calculator.
How it works
- Choose whether the relation is proportional or inverse.
- Enter the known pair, e.g. “3 kg correspond to 7.50 €”, and optionally the units.
- Enter the quantity whose value you are looking for.
- Read the result and the three-step calculation.
Frequently asked questions
How do I know whether it is proportional or inverse?
Ask yourself: if one quantity grows, does the other grow too? Then it is proportional (more goods cost more). If the other shrinks, it is inverse (more helpers need less time).
What is the formula?
Proportional: x = b ÷ a × c. Inverse: x = a × b ÷ c. For 4 workers needing 6 days, 3 workers need 4 × 6 ÷ 3 = 8 days.
Does the rule of three always apply?
Only if the relation really is (inversely) proportional. Volume discounts, base fees or group effects break it – then the result is only an approximation.
Can I convert units too?
For conversions between units such as km and miles or °C and °F, use the Unit Converter.