Proportion calculator

Find the missing value in A / B = C / X.

Your numbers

Both a decimal point and a comma work.

Result

100

How it works

A / B = C / X → X = B × C / A

Multiply B by C and divide by A. Zero values cannot define this proportion with X in the denominator.

Solving a proportion

For A / B = C / X, cross-multiply: A × X = B × C. Divide both sides by A to obtain X = B × C ÷ A. The displayed formula matches this exact arrangement of values.

Worked example

Suppose 100 / 250 = 40 / X. Then X = 250 × 40 ÷ 100 = 100. Substituting back gives 100 / 250 = 40 / 100 = 0.4.

When this calculation applies

Use this formula when two ratios are known to be equal. The current tool requires nonzero A, B and C; do not use it for proportions that require dividing by zero. Make sure the paired values use consistent units.