Proportion Calculator
Solve a proportion a ÷ b = c ÷ x for the missing term x.
How proportions work
A proportion states that two ratios are equal: a ÷ b = c ÷ x. If three of the four terms are known, the fourth can be found.
Enter A, B and C. The calculator solves for X using x = (B × C) ÷ A.
Proportions are the basis of scaling — recipes, map scales, unit conversions, sample splits. If one ratio is known and one term of the matching ratio is missing, a proportion finds it.
The formula
x = (B × C) ÷ A
Cross-multiply: a × x = b × c, then divide by a to get x = (b × c) ÷ a. The denominator A must not be zero.
Worked example
2 ÷ 4 = 6 ÷ x
x = (4 × 6) ÷ 2 = 12. So 2 ÷ 4 = 6 ÷ 12 — both ratios equal 0.5.
Frequently asked questions
When would I use a proportion?
Whenever two ratios are known to be equal and one term is missing — scaling a recipe, converting units, or estimating a count from a known rate.
Why can't A be zero?
A is the denominator of the first ratio. Dividing by zero is undefined, so the calculator rejects A = 0 rather than returning an invalid result.
Can the unknown X be negative?
Yes. If B or C is negative, X is negative. The relationship still holds mathematically.
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