Rule of Three Calculator
Solve a proportion instantly: if A corresponds to B, find what C corresponds to.
Result
Result (what C corresponds to)
10.00
How it works
The rule of three solves a proportion: if A corresponds to B, what corresponds to C? The answer is (B × C) ÷ A. If 3 kg of flour cost 4.50, then 7 kg cost (4.50 × 7) ÷ 3 = 10.50. It is probably the most-used piece of arithmetic outside a classroom. Scaling a recipe, converting a currency, pricing by weight, reading a map scale, adjusting a dose by body weight — all are the same operation wearing different clothes. Setting it up correctly is the only real skill: keep the units in matching positions. Flour goes above flour and money above money, or the answer comes out inverted. A quick sanity check catches most mistakes — if you asked for more, the answer must be bigger.
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Frequently asked questions
What is the rule of three?
It's a method to find an unknown value in a proportion. If A corresponds to B, then C corresponds to (B × C) ÷ A.
Can you give a practical example?
If 3 pens cost 6, how much do 5 pens cost? Set A = 3, B = 6, C = 5 → (6 × 5) ÷ 3 = 10.
When does the rule of three not apply?
Whenever the relationship is not proportional. One painter takes 6 hours, but six painters do not take one hour, and doubling a cake's size does not double its baking time. The rule needs a genuine straight-line relationship to be valid.
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