Proportion Calculator
Enter any three values of a : b = c : x and get the fourth — the classic cross-multiplication (rule of three) with a built-in validity check.
Rule: a × x = b × c, so x = (b × c) ÷ a. Zero a has no solution.
How to use
- Fill in a, b and c — leave the unknown as x.
- The answer x = b × c ÷ a appears instantly, with a cross-check shown.
- Change any input and the solution recalculates immediately.
Frequently asked questions
What does this proportion calculator solve?
It solves a : b = c : x — you enter any three known values and it computes the fourth, x = (b × c) / a. This is the classic “cross multiplication” rule, also known as the rule of three.
Can you show a real example?
If 3 pens cost $4.50, how much do 7 pens cost? Enter a=3, b=4.5, c=7 and x comes out 10.5 — so 7 pens cost $10.50. The same setup works for speeds, recipe scaling, fuel consumption and map scales.
Why can’t a be zero?
The formula divides by a. A proportion with a zero first term is degenerate: 0 : b = c : x would force b × x = 0 × c = 0, which has no unique solution when b ≠ 0. The tool shows an error instead of a wrong number.
Does it work with negative numbers or decimals?
Yes. Any finite real numbers work, including negatives and decimals — the cross-multiplication check (a × x = b × c) verifies the answer either way.
How is this different from a percentage calculator?
Proportions relate four quantities through a ratio; percentages relate a part to a whole on a 0–100 scale. Many percent problems can be set up as proportions (x/100 = part/whole), so the two tools overlap — pick whichever matches how you think.