Rule of Three Calculator
Online rule of three calculator for simple proportions. Enter three values to quickly find the fourth proportional value. Just fill in the fields and the calculation runs automatically, showing the result of the rule of three.
What is the rule of three?
The simple rule of three is a math method used to find an unknown value from a proportional relationship between other values. It is widely used in everyday situations such as working out prices, percentages, measurements and time.
How does the rule of three work?
When two quantities are proportional, you can set up a relationship between them to find an unknown value. In practice, you use three known values to find the fourth proportional value.
How to solve a rule of three
- Identify the known values — You have three values (A, B and C) and one unknown value (X).
- Set up the proportion — B corresponds to A just as C corresponds to X, so A ÷ B = X ÷ C.
- Cross-multiply — Multiply A by C.
- Divide by the remaining value — Divide the result by B to find X.
Rule of three example
If 20 items cost 10, how much do 30 items cost? With A = 10, B = 20 and C = 30, the formula is X = (A × C) ÷ B = (10 × 30) ÷ 20 = 15. This lets you find the proportional value quickly and easily.
Frequently asked questions
How do I solve a simple rule of three?
Set up the proportion so that B corresponds to A just as C corresponds to X. Cross-multiply and isolate X: X = A × C ÷ B.
What is the difference between direct and inverse proportion?
In direct proportion, when one quantity increases the other increases too (more items, higher price). In inverse proportion, when one increases the other decreases (more workers, less time).
Can I calculate percentages with the rule of three?
Yes, but the UtiliTools percentage calculator handles those cases more directly.