Proportion Calculator
Find the missing value in a proportion. Enter three known numbers and the calculator solves for the fourth, for both direct and inverse proportions.
Result
Direct proportion
Use it when two quantities increase or decrease together, written as A/B = C/X. If 3 pounds of apples cost $6, then 5 pounds cost X = 6 × 5 / 3 = $10. Cross-multiply the known values and divide by the remaining one.
Inverse proportion
Use it when one quantity goes up as the other goes down, so A × B = C × X. If 4 people take 6 days to finish a job, 3 people take X = 4 × 6 / 3 = 8 days.
Related guide: How to Solve Proportions (Direct and Inverse)
Frequently asked questions
How do I know if a proportion is direct or inverse?
Ask what happens when you double one quantity. If the other doubles too, it is direct. If it halves, it is inverse.
Can I use this to scale a recipe?
Yes. If a recipe for 4 servings needs 300 g of flour, enter A = 4, B = 300 and C = the number of servings you want.
What is cross-multiplication?
In A/B = C/X, multiplying across gives A × X = B × C, so X = B × C / A. It is the standard way to solve a direct proportion.
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Results are estimates for informational purposes only and are not financial, tax, legal or medical advice.