Proportion Calculator
Solve a proportion a/b = c/d when exactly one value is unknown
Missing value
How we calculated it
About the Proportion Calculator
Proportion Calculator solves relationships where two ratios are equal. It is useful for scaling recipes, maps, dimensions, prices and rates when three values are known and a fourth must be found.
The quantities compared should be arranged consistently. Reversing only one ratio can create a plausible-looking but incorrect answer.
How to Use the Proportion Calculator
- Enter the three known values in the proportion a/b = c/d.
- Identify which term is unknown.
- Calculate the missing value.
- Check that corresponding quantities occupy matching positions in both ratios.
How the Calculation Works
For a/b = c/d, cross multiplication gives ad = bc.
The unknown term is isolated by dividing the known product by the remaining coefficient, provided the required denominator is non-zero.
Worked example
Proportion Calculator Worked Example
3/5 = x/20
Cross multiply: 5x = 3×20 = 60. Therefore x = 60÷5 = 12.
Result guide
Understanding Your Proportion Calculator Result
A solved proportion preserves the same multiplicative relationship between corresponding quantities. It does not establish that the relationship should be proportional in the real-world problem. Proportions are especially useful when a constant scale factor is known, but they should not be used for relationships that are curved, stepped or subject to minimum fees and fixed costs.
Before you rely on the answer
Assumptions and Limitations
- Ratios represent corresponding quantities in the same order.
- Necessary denominators are non-zero.
- A proportional relationship is appropriate for the problem.
Practical tip
Using This Result in Practice
Write units beside each term before solving. If kilograms appear on the top of one ratio and the bottom of the other, the proportion is probably arranged incorrectly. If the answer has a unit, carry that unit through the cross-multiplication check so a misplaced term is easier to spot.
Related calculations
Useful Calculations to Continue With
If you first need to simplify or compare two ratios, use the Ratio Calculator.
If the relationship is more naturally expressed per 100, use the Percentage Calculator.
Common questions
Proportion Calculator FAQ
How do you solve a proportion?
Cross multiply the equal ratios and solve the resulting equation for the unknown.
Can a proportion have zero in the denominator?
No. Division by zero is undefined.
How do I know the ratios are arranged correctly?
Keep corresponding units or quantities in the same numerator/denominator positions.