What is the rule of three?
The rule of three is a quick way to solve a proportion — a statement that two ratios are equal. If you know three of the four values, you can find the fourth. It is one of the most useful pieces of everyday maths: scaling a recipe, converting prices, working out fuel or paint quantities, or any 'if this, then how much of that' question.
How it works
You set it up as A is to B as C is to X. To find X you cross-multiply: X = B × C ÷ A. For example, if 3 apples cost 12, then 5 apples cost 12 × 5 ÷ 3 = 20. This is direct proportion, where increasing one side increases the other.
Examples
- Recipe: 3 eggs make 12 cookies → 5 eggs make 20.
- Price: 2 kg costs 9 → 7 kg costs 31.50.
- Speed: 60 km in 45 min → 80 km in 60 min.
Frequently asked questions
Direct or inverse proportion?
This calculator does direct proportion (both grow together). Inverse is a different setup.
What if A is zero?
A can't be zero — you can't divide by it. Enter a non-zero known value.
Is my data private?
Yes — everything is calculated in your browser.