Rule of Three Calculator
A free, browser-based calculator. Runs entirely in your browser — no sign up, nothing stored.
100% FreeRuns in your browserBasic
Enter Values
Enter your values and press Calculate to see results.
How to use this calculator
- Enter the three known values A, B and C.
- Press Calculate to find X, where A is to B as C is to X.
How it works
For a direct proportion A : B = C : X, the unknown is X = B × C ÷ A. It's the classic cross-multiply method for scaling quantities.
Worked example
3 : 12 = 5 : X. X = 12 × 5 ÷ 3 = 20.
Frequently asked questions
What's the rule of three used for?
Scaling recipes, converting quantities, working out unit prices, map distances — anything where two amounts stay in proportion.
Related tools
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.