Rule of Three Calculator

Enter A, B and C in the classic 'if A corresponds to B, what does C correspond to?' problem to solve for D.

Rule of Three Calculator

Solve a rule-of-three (direct proportion) problem: if A corresponds to B, find what C corresponds to.

This calculator provides an estimate for general informational purposes only. It is not a substitute for professional financial, mechanical or engineering advice.

The Rule of Three is a classic method for solving proportional problems — given three related values, it finds the fourth by cross-multiplying and dividing.

Example

  • With the default values shown above, this calculator returns: D (Corresponds to) ≈ 32.00.

Frequently Asked Questions

What is the Rule of Three?

The Rule of Three solves direct proportion problems: if A relates to B the same way C relates to D, then D = (B × C) ÷ A.

What's a real example?

If 5 apples cost $20, how much do 8 apples cost? Here A=5, B=20, C=8, and D = (20×8)÷5 = $32.

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