Proportion Calculator
Enter three known values below to instantly solve the proportion for x.
Solve a/b = c/x for x
Solving a Proportion
For a/b = c/x, cross-multiply: x = (b × c) ÷ a.
A common mistake is cross-multiplying the wrong diagonal pair — multiplying the two numerators together instead of a numerator by the opposite denominator. Cross-multiplying always pairs the numerator of one fraction with the denominator of the other, never numerator with numerator.
Worked example
Solve 3/4 = 9/x.
x = (4 × 9) ÷ 3 = 36 ÷ 3
x = 12
A second example: solve 5/8 = x/24.
5 × 24 = 8 × x, so x = 120 ÷ 8
x = 15
Keep going
- A proportion is just two ratios set equal to each other; the ratio calculator simplifies a single ratio to its lowest terms first. Ratio Calculator
- Speed, distance, and time relate through direct proportion; the speed distance time calculator applies that same a/b = c/x relationship to a real-world rate problem. Speed, Distance, Time Calculator
Frequently Asked Questions
How do you solve a proportion?
Cross-multiply: for a/b = c/x, that gives a × x = b × c, so x = (b × c) ÷ a.
What is a proportion in math?
A proportion is a statement that two ratios are equal, written as a/b = c/d. Solving one means finding a missing value that keeps both ratios equivalent.
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?) or converting a scale drawing to real-world size are both proportion problems.
Why does cross-multiplication actually work?
Starting from a/b = c/d, multiplying both sides by b and by d clears both denominators at once: (a/b) × bd = (c/d) × bd simplifies to a × d = c × b. Cross-multiplication is just that algebra step done in one move, not a separate trick.
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