Proportion Calculator (Solve for X)
Solve for the missing value in an A:B = C:D proportion, with the cross-multiplication steps spelled out.
Give any three values in an A:B = C:D proportion and this tool finds the fourth. It solves for X using cross multiplication — the product of the means equals the product of the extremes — and shows each step of the work, so it's just as useful for learning the method as for getting a fast answer.
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How to use
- Use the dropdown at the top to pick which of A, B, C, or D is your unknown X.
- Fill in the remaining three fields with the values you already have.
- The value of X that makes the proportion true is calculated and displayed instantly below.
- Review the step-by-step work underneath to see exactly how cross multiplication solves the equation.
FAQ
- What's the core rule behind proportions?
- In any proportion, the product of the two outer terms (the extremes, A and D) always equals the product of the two inner terms (the means, B and C) — that is, A × D = B × C. Rearranging that equation isolates whichever term is unknown.
- Can I enter decimals or fractions?
- Decimals can be typed straight into the fields and are handled with full precision. For a fraction, just enter its decimal equivalent.
- Can I enter zero or a negative number in one of the ratio fields?
- Zero can be a problem depending on where it lands — if it ends up acting as a denominator (the B or D term), the equation becomes mathematically undefined and returns an error. Negative numbers are arithmetically fine, but since most real-world ratios (prices, distances, and so on) don't have a meaningful negative interpretation, it's worth double-checking why you'd need one before entering it.