Rule of three calculator
Three values known, one missing? Enter what you know, pick “more means more” or “more means less” – the calculator shows the result and the working.
How it works
Pick the kind
Direct: twice the goods cost twice as much. Inverse: twice as many workers need half the time.
Enter the values
At the top the known pair (e.g. 3 kg → 4.50), at the bottom the new value (5 kg). Units are optional and show up in the working.
Read the working
The rule of three in three steps: the known pair, down to one unit (1) and up to the value you need.
Frequently asked questions
How does the rule of three work?+
You go from a known pair via one unit to the value you need. Example: 3 kg cost 4.50. 1 kg costs 4.50 ÷ 3 = 1.50. 5 kg cost 1.50 × 5 = 7.50.
When is it an inverse proportion?+
When more of one quantity means less of the other: 4 painters need 6 days, 1 painter needs 4 × 6 = 24 days, 3 painters need 24 ÷ 3 = 8 days. Going down to one unit you multiply instead of dividing.
How do I do it quickly in my head?+
Direct: value = b₁ ÷ a₁ × a₂. Inverse: value = b₁ × a₁ ÷ a₂. The calculator shows both with your numbers.
What is the rule of three used for?+
Prices and quantities when shopping, recipes for more people, fuel consumption, working time, currencies, map scales and many tasks in school, training and aptitude tests.