Proportion Calculator

A proportion says two ratios are equal, such as 3/4 = 15/x. To find the missing value, cross-multiply: multiply diagonally across the equals sign and divide. This calculator solves for x exactly, with fractions where needed, and checks that both ratios really are equal. It is useful for recipes, maps and scaling.

Solves a / b = c / x for x.
Try:

How to use the proportion calculator

  1. Enter a, b and c from a/b = c/x.
  2. Press Calculate.
  3. Read x and the cross-multiplication steps.

Formula

ab=cx⇒x=bca\frac{a}{b} = \frac{c}{x} \Rightarrow x = \frac{bc}{a}

Worked example: 3/4 = 15/x

  • a: 3
  • b: 4
  • c: 15

✓ Answer checked

x=20x = 20
Step-by-step working (3 steps)

  1. Write the proportion

    Two equal ratios.

    34=15x\frac{3}{4} = \frac{15}{x}

  2. Cross-multiply Cross-multiplication

    Multiply diagonally across the equals sign.

    3 x=4×15=603\,x = 4 \times 15 = 60

  3. Divide

    Divide both sides by 3.

    x=20x = 20
Formulas used
ab=cx⇒x=bca\frac{a}{b} = \frac{c}{x} \Rightarrow x = \frac{bc}{a}
How this was checked
  • ✓ Both ratios are exactly equal with this value of x.

Frequently asked questions

How do I solve 3/4 = 15/x?

Cross-multiply: 3x = 4 × 15 = 60, so x = 20.

How do I scale a recipe?

If 2 cups serve 5 people, set 5/2 = 8/x for 8 people: x = 2 × 8 ÷ 5 = 3.2 cups.

What is the difference between a ratio and a proportion?

A ratio compares two amounts; a proportion states that two ratios are equal.

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