How to use this tool
- Type three of the four numbers in the proportion a/b = c/d.
- Leave the unknown box empty.
- Read the answer and the cross-multiplication working.
Cross multiplication
Two ratios are in proportion when they are equal, as in 3/4 = 9/12. Multiplying diagonally gives the same product on both sides: 3 × 12 = 4 × 9 = 36. If one number is unknown, divide the known diagonal product by the number opposite the gap.
Everyday uses
Scaling a recipe from 4 servings to 6, working out real distances from a map scale, converting currency at a fixed rate, or comparing price per unit all use proportions. In medicine, dose-per-weight sums are proportions too, though clinical doses should always come from the prescriber or the official label.
Ratios and percentages
A percentage is a proportion with 100 as the denominator: 45/60 = x/100 gives 75%. The percentage calculator handles those directly.
Frequently asked questions
How do you solve a proportion?
Cross multiply and divide. For a/b = c/x, x = b × c ÷ a.
What is 3 is to 4 as 9 is to what?
12, because 3/4 = 9/12.
How do I scale a recipe?
Set original servings / new servings = original amount / new amount and solve for the new amount.
Can I use decimals?
Yes. Any numbers work except dividing by zero.