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Ratio & Proportion Calculators

Proportion Calculator

Solve proportions A/B = C/D for the missing value using cross-multiplication, with steps.

Enter values

Results update as you type.

Result

D

12

Step-by-stepShow
  1. 3/4 = 9/D
  2. D = (4 × 9) ÷ 3 = 12

About the Proportion Calculator

Solve proportions A/B = C/D for the missing value using cross-multiplication, with steps.

Also known as: cross multiplication calculator, solve for x proportion, rule of three, direct proportion, ratio proportion solver.

How to use this calculator

  1. Enter a, b, c, solve for.
  2. The result updates instantly as you type — press Calculate (or Enter) at any time.
  3. Open “Step-by-step” to see how the answer was worked out.
  4. Use Copy result or Share to save the answer or send a link with your inputs.

Formula

A/B = C/D ⇒ D = B × C ÷ A

Worked example

Using the values pre-filled in the calculator above:

  • A = 3
  • B = 4
  • C = 9
  • Solve for: D (A/B = C/D)

the calculator returns:

  • D: 12

Working

  1. 3/4 = 9/D
  2. D = (4 × 9) ÷ 3 = 12

Frequently asked questions

What is the rule of three?

If A corresponds to B, then C corresponds to B × C ÷ A. It is the everyday name for solving a direct proportion, e.g. 3 apples cost $4, so 9 apples cost $12.