Proportion Calculator
Type any three of the four numbers in a proportion (a : b = c : d) and this calculator solves for the missing one.
Enter 3 of the 4 values (a : b = c : d) and leave the one you want to find empty.
How to solve a proportion by hand
- Write the proportion as two equal fractions: a/b = c/d.
- Cross-multiply: multiply diagonally across the equals sign, giving a × d = b × c.
- Divide both sides by the number next to the unknown to leave it on its own.
Exemple résolu
Solve 2 : 3 = 10 : x. Cross-multiply: 2 × x = 3 × 10, so 2x = 30, and x = 15.
Solve proportions, percentages and every other formula with your own numbers in Formules mathématiques.
Foire aux questions
What is a proportion in maths?
A statement that two ratios are equal, written a : b = c : d (or a/b = c/d). It means the two pairs of numbers scale by the same factor.
How do you solve a proportion for a missing value?
Cross-multiply: a/b = c/d means a × d = b × c. Then divide both sides by whichever number is next to the unknown to isolate it.
What is the cross-multiplication rule?
For a/b = c/d, multiply diagonally: a × d = b × c. This turns the proportion into a simple one-step equation for the missing value.
If 2 is to 3 as 10 is to what number?
Set up 2/3 = 10/x. Cross-multiply: 2x = 30, so x = 15.
