FractionReference

Solve a proportion

Fill in three boxes and leave the unknown one empty. The two ratios are held equal, and the missing term follows from cross multiplication.

Try:

Common questions

How do I solve a proportion?
Cross multiply. In a : b = c : x, the products a × x and b × c are equal, so x is b × c divided by a. Leave the unknown box empty and this page does that and shows the working.
What is a proportion used for?
Scaling anything that keeps its shape: recipes, drawings, maps, mixtures. If 3 cups of flour serve 4 people, then 3 : 4 = x : 10 answers how much flour serves ten.
Can I put fractions in the boxes?
Yes. Terms can be whole numbers, fractions, mixed numbers or decimals — 1 1/2 : 2 = x : 8 solves exactly, without turning anything into a rounded decimal first.

Related

Simplify a ratioReduce a ratio to its lowest whole numbers. Equivalent fractionsA proportion is two equivalent fractions. Compare fractionsCross multiplication, used to rank instead of solve.
Where this comes from. Exact rational arithmetic on the values shown — fractions are pairs of integers until display, and inch–metric conversion uses 25.4 mm to the inch, exact by definition since 1959. See the full methodology.