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Proportion Calculator

Use this free online proportion calculator to solve for the missing value in A/B = C/D. Enter three known values and get the unknown instantly using cross multiplication.

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Formula

If A/B = C/D, then D = (B × C) / A

Cross-multiply and divide to find the missing value. A × D = B × C.

Worked Example

Solve: 3/4 = 9/x Step 1: Cross multiply → 3 × x = 4 × 9 Step 2: 3x = 36 Step 3: x = 36 / 3 = 12 Result: x = 12

What Is a Proportion?

A proportion is a mathematical statement that two ratios are equal. If you know three of the four values in a proportion, you can find the missing one using cross multiplication. Proportions are one of the most practical tools in everyday mathematics.

If 2 apples cost $3, then 6 apples cost $9 because 2/3 = 6/9
Cross multiplication works by setting the products of the diagonals equal: A x D = B x C
Proportions are the basis for unit conversions, map scales, and recipe scaling
Any equivalent fractions form a proportion: 1/2 = 3/6 = 50/100

Proportions are used daily in cooking, construction, medicine (dosage calculations), currency conversion, and anywhere you need to scale a known relationship to a new size.

You can also calculate changes using our Ratio Calculator, Fraction Calculator or Percentage Calculator.

Frequently Asked Questions

What is a proportion?

A proportion is an equation stating that two ratios are equal. For example, 1/2 = 3/6 is a proportion because both ratios simplify to the same value.

How do I solve a proportion?

Use cross multiplication: multiply the means and extremes, then solve for the unknown. In A/B = C/D, multiply A x D and B x C, set them equal, and solve.

When are proportions used?

In recipes (scaling ingredients), maps (converting scale distances to real distances), unit conversions, medicine (dosage by weight), and many other real-world applications.

What is cross multiplication?

Cross multiplication is a shortcut for solving proportions. In A/B = C/D, you multiply diagonally: A x D = B x C. Then solve for the unknown variable.

Can proportions have more than two ratios?

Yes. Extended proportions like A:B:C = D:E:F compare three or more quantities simultaneously. Each pair of corresponding values must maintain the same ratio.

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