Proportion Calculator
Enter three of the four values in the proportion a/b = c/d and leave the unknown blank. The calculator cross multiplies to solve for the missing number instantly.
How to solve a proportion
- Fill in three of the four boxes (a, b, c, d).
- Leave exactly one box blank for the unknown.
- Read the solved value, found by cross multiplication.
Examples
Solve for c
2 / 4 = c / 10
c = 5
Solve for a
a / 4 = 5 / 10
a = 2
Frequently asked questions
What is a proportion?
A proportion states that two ratios are equal, written a/b = c/d. If three of the four values are known, the fourth can always be found.
How does cross multiplication work?
From a/b = c/d we get a x d = b x c. Rearranging that equation solves for whichever value is missing.
Which box should I leave blank?
Leave exactly one of a, b, c or d empty. That blank is treated as the unknown the calculator solves for.
Why do I get a division by zero error?
Solving for a value can require dividing by another. If the divisor you need is zero, the proportion has no finite solution.
Is my data private?
Yes. The calculation runs in your browser and nothing is uploaded.
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