Proportion Calculator

Solve proportions in the form a/b = c/x.

Proportion Values	Enter Value
a
b
c

Enter a, b and c to solve a/b = c/x.

How to Use the Proportion Calculator

Enter a, b and c for the proportion a/b = c/x. The calculator solves the missing value with x = (b × c) ÷ a.

Practical Example

If 2/5 = 8/x, then x = (5 × 8) ÷ 2 = 20.

Factors That Affect the Result

The calculation assumes the two ratios are truly proportional. Rounding, inconsistent units and nonlinear relationships can make a mathematically correct result inappropriate in practice.

Frequently Asked Questions

What is a proportion?

It is an equation stating that two ratios are equal.

Can I use decimal values?

Yes, provided the values represent a valid proportional relationship.

Why can a not be zero?

The formula divides by a, so a value of zero would make the calculation undefined.

Solving Proportions

A proportion states that two ratios are equal, such as a/b = c/d. When three values are known, the fourth can often be found by cross multiplication. For example, if 3/5 = x/20, then 5x = 60 and x = 12. The denominator of a ratio cannot be zero. Proportions are useful for scaling recipes, maps, drawings, prices, mixtures and many other comparisons where the same relative relationship is maintained. Make sure the quantities being compared are compatible and place corresponding units in the same positions in each ratio. If one ratio is expressed in centimeters and another in meters, convert units before solving or account for the conversion explicitly. A mathematically correct proportion can still be inappropriate if the real-world relationship is not actually linear, so consider whether proportional scaling is a reasonable assumption for the problem.