Proportion Calculator

Solve a/b = c/x for whichever value is missing.

Proportion Calculator

Worked example result

Missing value	6.00
Completed proportion	2/4 = 3/6

2/4 = 3/x gives x = (4 × 3) ÷ 2 = 6. Check it: both sides equal 0.5, so the proportion 2/4 = 3/6 is true.

A proportion states that two ratios are equal: a/b = c/d. If you know any three of the four values, the fourth is fixed — this calculator finds it by cross-multiplication.

Choose which value is missing, fill in the other three, and the missing value is solved for you. Proportions underpin scale drawings, unit conversions, similar triangles and recipe scaling.

The formula

a/b = c/d, solved by cross-multiplication: the missing value makes a × d = b × c

Worked example

2/4 = 3/x gives x = (4 × 3) ÷ 2 = 6. Check it: both sides equal 0.5, so the proportion 2/4 = 3/6 is true.

The example is pre-loaded in the result panel — press Calculate with your own numbers to replace it.

Frequently asked questions

What is cross-multiplication?

For a/b = c/d, cross-multiplication says a × d = b × c. Solving for the unknown is then one division: for example d = (b × c) ÷ a. It works because multiplying both sides by b × d clears both denominators.

Where are proportions used in real life?

Scaling recipes, reading maps (1 inch = 50 miles), converting currencies and units, enlarging photos without distortion, and dosing medicine by body weight all rely on proportions.

Category: Math calculators

Proportion Calculator