Rule of Three Calculator

Enter three proportional values A, B, and C to solve for the missing value X in the relationship A : B = C : X.

Represents the known quantity paired with B.
Keep the same units you want the answer in (cost, weight, etc.).
The quantity that needs a proportional equivalent of B.
Optional: choose how many decimal places to display (0–6).

Educational information, not professional advice.

Examples

  • 2 books cost $10 ⇒ 5 books cost $25.00
  • 3 kg correspond to $12 ⇒ 7 kg cost $28.00
  • 4 hours deliver 360 widgets ⇒ 9 hours deliver 810 widgets

FAQ

What is the rule of three?

It solves a proportion A : B = C : X by cross-multiplication, giving X = (B × C) ÷ A.

Can I use decimals or percentages?

Yes. Enter decimals directly or convert percentages to decimals (e.g., 12% = 0.12) before calculating.

Why must A not be zero?

Division by zero is undefined, so the base quantity A must be nonzero.

Do the inputs need units?

Use consistent units for B and C so the result inherits B’s unit. Mixing units breaks the proportion.

Additional Information

  • Solve proportions with x = (B × C) ÷ A when A, B, and C are in matching ratios.
  • Keep B and the result X in the same unit (currency, weight, time) for interpretable answers.
  • If the situation is inversely proportional, invert the relationship before applying the rule of three.

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