Proportion Calculator
Solve a proportion a/b = c/d for whichever term you leave blank.
Fill in three of the four values in a / b = c / d and leave the unknown blank.
Solved for c
6
c makes the two ratios equal.
How it works
A proportion says two ratios are equal, like a/b = c/d. If you know three of the four numbers, the fourth is pinned down. Leave that one field empty and the calculator fills it in.
The trick is cross-multiplication: a/b = c/d is the same as a × d = b × c. Rearranging for whichever term is missing gives it directly. To find c when a = 2, b = 3, and d = 9, you compute (2 × 9) ÷ 3 = 6.
This is the everyday tool for scaling recipes, converting units, and reading map distances. Just watch the zeros — a denominator of zero has no solution, so you'll get a dash instead.
Frequently asked questions
How do I set up a word problem as a proportion?
Keep the same kinds of quantity across from each other. If 3 apples cost $2 and you want the price of 12 apples, write 3/2 = 12/x — apples over dollars on both sides — then solve for x.
What if I leave more than one box empty?
A single missing value has one answer, but two blanks leave infinitely many. So the calculator needs exactly three numbers filled in; with fewer, it shows a dash until you complete them.
Why did I get a dash instead of a number?
That usually means a division by zero snuck in — for example the term you're solving through is zero, which makes the ratio undefined. Adjust the values so no denominator lands on zero.