How do I solve a proportion?

For a/b = c/x, cross-multiply to get a·x = b·c, then x = b·c/a. For 2/3 = 8/x, x = 3·8/2 = 12.

What can this be used for?

Scaling recipes, converting units, map distances, similar triangles — anywhere two ratios are equal and one term is unknown.

Proportion Solver

Free proportion solver. Enter three values of the proportion a/b = c/x and get the missing value x by cross-multiplication (x = b·c/a), plus the ratio a/b and the cross product.

Solve a/b = c/x for the unknown value x by cross-multiplication.

\frac{a}{b} = \frac{c}{x} \Rightarrow x = \frac{b \cdot c}{a}

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