Tool Factory

Proportion Calculator

Solve a proportion A / B = C / D for the missing value, or check whether two ratios are proportional. See the cross products and the scale factor. Everything runs in the browser.

  • Solve for x
  • Check proportions
  • Cross products & scale factor
  • Stays on your device
Mode

A / B = C / D. Leave one box blank and the calculator cross-multiplies to solve for it.

Numbers stay on this device. Nothing is uploaded, and there is no account.

A proportion says two ratios are equal: A / B = C / D. Because the cross products of a true proportion match (A × D = B × C), you can find any one missing term by multiplying the diagonal pair you know and dividing by the remaining number.

Example: a map scale of 3 cm to 4 km means 15 cm on the map covers (4 × 15) ÷ 3 = 20 km. To check a pair of ratios, compare the cross products: 2 / 3 and 10 / 15 are proportional because 2 × 15 and 3 × 10 both equal 30.

Need to reduce a single ratio to lowest terms instead? Use the ratio calculator. Browse more tools in the Math hub, find the greatest common factor with the GCF calculator, factor one number with the factoring calculator, or work out a share of a total with the percent off calculator.

How to use the proportion calculator

  1. Pick Solve for the missing value or Check if ratios are proportional.
  2. To solve, fill in three of A, B, C, and D and leave the unknown blank.
  3. To check, enter all four numbers.
  4. Read the answer, the cross products A × D and B × C, and the scale factor.

Frequently asked questions

What is a proportion calculator?

A proportion calculator works with two equal ratios written as A / B = C / D. It solves for one missing value when you know the other three, or checks whether four numbers form a true proportion.

How do I solve a proportion?

Cross-multiply, then divide. In A / B = C / D, the cross products are equal: A × D = B × C. For 3 / 4 = 15 / x, 3 × x = 4 × 15, so x = 60 ÷ 3 = 20.

How do I know if two ratios are proportional?

Multiply diagonally. If A × D equals B × C, the ratios are proportional. 2 / 3 and 10 / 15 are proportional because 2 × 15 = 30 and 3 × 10 = 30. 2 / 3 and 10 / 14 are not, because 28 does not equal 30.

What is the cross multiplication formula?

For A / B = C / D, the formula is A × D = B × C. Rearranged: A = (B × C) ÷ D, B = (A × D) ÷ C, C = (A × D) ÷ B, and D = (B × C) ÷ A.

What is the scale factor in a proportion?

The scale factor is how much the first ratio is multiplied to get the second. In 3 / 4 = 15 / 20 both parts are multiplied by 5, so the scale factor is 5. It equals C ÷ A (or D ÷ B).

What is the difference between a proportion and a ratio?

A ratio compares two numbers, like 3 : 4. A proportion is an equation stating that two ratios are equal, like 3 : 4 = 15 : 20. Use the ratio calculator to simplify a single ratio, and the proportion calculator to solve or check an equation of two ratios.

Can a proportion have decimals, fractions, or zero?

Decimals work, such as 1.5 / 4 = x / 12, which gives x = 4.5. Convert fractions to decimals first. B and D are denominators, so they cannot be zero, and the value you divide by when solving cannot be zero either.

Is my information saved or uploaded?

No. The proportion calculator runs entirely in your browser. Numbers are not sent to a server or stored, and there is no account.

Other tools