Proportion Calculator

Solve for X in proportional equations using cross multiplication.

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Enter 3 values. Leave the X field empty (or type 0).
X =
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How to Solve Proportions

A proportion states that two ratios are equal. For example, if you have a recipe that serves 4 people and you want to serve 10, you can use a proportion to scale the ingredients.

The Cross-Multiplication Method

The easiest way to solve for X is to use cross-multiplication.

Given: A / B = C / D
Multiply diagonally: (A × D) = (B × C)
Isolate X and divide.

Common Examples

  • Map Reading: 1 inch = 10 miles. How many miles is 3 inches? (1/10 = 3/X)
  • Shopping: 5 apples cost $2. How much do 8 apples cost? (5/2 = 8/X)
  • Speed: 60 miles in 1 hour. How many miles in 2.5 hours? (60/1 = X/2.5)