Rule of Three Calculator

Solve a direct proportion using three known values.

Proportion Values	Enter Value
A
B
C

Enter A, B and C to solve for X.

How to Use the Rule of Three Calculator

Enter the three known proportional values A, B and C. The calculator solves X using X = (B × C) ÷ A.

Practical Example

If 4 notebooks cost USD 12, then 10 notebooks at the same rate cost (12 × 10) ÷ 4 = USD 30.

Factors That Affect the Result

The method assumes a direct linear relationship. If discounts, fixed fees, taxes, tiered pricing or nonlinear behavior apply, the proportional result may not match the real outcome.

Frequently Asked Questions

What is the rule of three?

It is a shortcut for solving one unknown value when two ratios are proportional.

Can I use decimals?

Yes, provided the values represent a valid direct proportion.

When should I not use it?

Avoid it when the relationship is not linear or when fixed charges and thresholds affect the result.

Rule of Three Calculator – Solve Proportional Values