Proportion Calculator
Find the missing value in a proportion.
Missing value X
A must be nonzero.
Formula checked · NumberSwift · Review process
How this calculator works
A proportion states that two ratios are equal. Cross multiplication lets you solve for the missing fourth value without changing the relationship. Enter A, B and C in A/B = C/X. The first value must be nonzero because solving for X divides by A.
Formula
A / B = C / X → X = B × C / A
Assumptions & limitations
A must be nonzero.
Worked example
2/3 = 8/12.
If all you need is the simplest whole-number relationship, use the Ratio Calculator instead of solving a fourth value.
Our approach to calculations ↗Common questions
Where does the missing value go?
The layout is A/B = C/X. Enter the first three values and the tool solves for X.
Why can A not be zero?
Solving for X uses B × C / A, so a zero A would require division by zero.
Can I scale a recipe or drawing?
Yes, when the quantities are proportional and units are consistent. The tool assumes that relationship rather than proving it.