Proportion Calculator
Solve proportions by finding the missing value in A/B = C/D. See also Cross Multiplication Calculator and Ratio Calculator.
How to Solve Proportions
A proportion states that two ratios are equal: A/B = C/D. To find a missing value, use cross multiplication. Multiply the two diagonal values that are known, then divide by the remaining known value. This method works for any proportion where exactly one value is unknown.
Proportion Formula
A / B = C / D
Cross multiply: A × D = B × C
Solve for unknown:
A = (B × C) / D
B = (A × D) / C
C = (A × D) / B
D = (B × C) / A
Example
Solve: 3/6 = ?/12
Cross multiply: 3 × 12 = 6 × ?
36 = 6 × ?
? = 36 / 6 = 6
Check: 3/6 = 6/12 = 0.5 ✓
Frequently Asked Questions
What is a proportion?
A proportion is an equation that states two ratios are equal. For example, 1/2 = 3/6 is a proportion because both ratios simplify to the same value (0.5).
How is a proportion different from a ratio?
A ratio compares two quantities (e.g., 3:4), while a proportion states that two ratios are equal (e.g., 3/4 = 6/8). A proportion always involves four values.
Can proportions have negative numbers?
Yes, proportions can include negative numbers. The cross multiplication method works the same way. Just be careful with signs when multiplying and dividing.
What if the denominator is zero?
Division by zero is undefined, so neither B nor D can be zero in a proportion A/B = C/D. If solving for B or D, the result will be zero only if the corresponding numerator is also zero.