Cross Multiplication Calculator
Solve any proportion using cross multiplication. Enter three values to find the missing fourth, or enter all four to verify whether the proportion is true. A fundamental tool for algebra and ratio problems.
Cross multiplication is the standard technique for solving proportions. Given a/b = c/d, cross multiplication states that a x d = b x c. This transforms a proportion into a simple equation.
How Cross Multiplication Works
Multiply the numerator of each fraction by the denominator of the other. If these cross products are equal, the proportion is valid. If one value is unknown, solve the resulting equation.
For example, 3/4 = x/20: cross multiply to get 3 x 20 = 4 x x, so 60 = 4x, giving x = 15.
When to Use Cross Multiplication
Use it whenever you have a proportion (two equal ratios) with one unknown. Common applications include unit conversions, recipe scaling, map distances, similar triangles, and solving percent problems.