Cross Multiplication Calculator
Solve A/B = C/D using cross multiplication with step-by-step solution. See also Proportion Calculator and Fraction Calculator.
How to Use Cross Multiplication
Cross multiplication is a method for solving proportions. Given A/B = C/D, multiply diagonally: A × D = B × C. Then isolate the unknown variable by dividing both sides by the coefficient next to it. This technique works whenever you have two equal fractions with one unknown value.
Cross Multiplication Formula
Given: A/B = C/D
Cross multiply: A × D = B × C
To find A: A = (B × C) / D
To find B: B = (A × D) / C
To find C: C = (A × D) / B
To find D: D = (B × C) / A
Example
Solve: 3/4 = x/8
Step 1: Cross multiply → 3 × 8 = 4 × x
Step 2: 24 = 4x
Step 3: x = 24 ÷ 4
Step 4: x = 6
Verify: 3/4 = 6/8 = 0.75 ✓
Frequently Asked Questions
Why does cross multiplication work?
Cross multiplication works because multiplying both sides of A/B = C/D by B × D eliminates the denominators, giving A × D = B × C. This is a valid algebraic operation that preserves the equality.
Can I use cross multiplication with more than two fractions?
Cross multiplication only works directly with two fractions set equal to each other. For chains of equal fractions (A/B = C/D = E/F), solve them in pairs.
What if the answer is a fraction?
The result of cross multiplication can be any real number, including fractions and decimals. The calculator shows the exact decimal result.