📏 Rule of Three Calculator
Solve direct and inverse proportions instantly by entering just three known values.
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Enter the three known values
How to use the rule of three
The rule of three is a method to solve proportions when three of the four values are known. It is one of the most practical mathematical tools in everyday life, used for calculating prices by quantity, scaling recipes, converting units, calculating work times, reading map scales and much more.
Direct proportion example: If 3 kg of apples cost £7, how much do 5 kg cost? Answer: (7 × 5) ÷ 3 = £11.67
Inverse proportion example: If 4 workers finish a job in 6 days, how long would 1 worker take? Answer: (4 × 6) ÷ 1 = 24 days