Proportion Calculator: Solve A/B = C/D Instantly
A proportion states that two ratios are equal — A/B = C/D. Given any three of the four values, cross-multiplication lets you solve directly for the fourth. This calculator handles all four possible unknowns and shows the result immediately as you type.
Rearrange for whichever variable is unknown; division by zero has no solution.
Why Cross-Multiplication Works
Multiplying both sides of A/B = C/D by B and by D clears both denominators, leaving A×D = B×C — a simple equation with no fractions, easily solved for any single unknown by dividing both sides by whatever coefficient remains next to it.
Common Uses for Proportions
Recipe scaling, unit conversion, map scale distances, similar-triangle geometry problems, and financial ratios (price-per-unit comparisons) are all direct applications of solving a proportion for an unknown quantity.
Practical Examples
Scaling a Recipe
2 cups flour makes 12 cookies — how much for 30?
- 1.A=2, B=12, C=x, D=30
- 2.Solve for C: 5 cups
Good Use Cases
- Recipe and batch-size scaling
- Similar-triangle and scale-drawing geometry
- Unit rate and price comparisons
- Quick algebra homework checks
Frequently Asked Questions
What is cross-multiplication?
For a proportion A/B = C/D, cross-multiplication says A×D = B×C. Rearranging that equation for whichever variable is unknown gives you a direct formula to solve for it.
What if one of my known values is zero?
A proportion A/B = C/D requires B and D to be nonzero (you can't divide by zero), and if the value you're solving for depends on dividing by a known value that's zero, no solution exists — the calculator will show no result in that case.
Can I solve for any of the four positions, not just D?
Yes — select which letter (A, B, C, or D) is unknown, then fill in the other three.
What's a real-world example of a proportion?
Scaling a recipe: if 2 cups of flour makes 12 cookies, how much flour makes 30 cookies? Set up 2/12 = x/30 and solve for x.
Does this handle non-integer results?
Yes, results are shown as decimals when the division doesn't come out evenly.