Rule of Three Calculator
Fill in A, B and C to find the missing value X in a direct or inverse proportion.
A (2) is to B (10) as C (5) is to X
X
25
Direct vs. inverse proportion
In a direct proportion, A/B = C/X, so X = (B × C) / A. In an inverse proportion, A × B = C × X, so X = (A × B) / C. Pick the type that matches how your two quantities relate to each other.
Frequently asked questions
What is a direct rule of three?
It applies when two quantities grow together — as A increases, C increases too, such as price per item scaling with quantity.
What is an inverse rule of three?
It applies when one quantity increases while the other decreases — such as more workers finishing a job in less time.
How is X calculated?
For a direct proportion, X = (B × C) / A. For an inverse proportion, X = (A × B) / C.
Related tools
You might also need these while working on the same task.
Calculate percentage of a value, percentage change, and discounted price.
Open toolCalculate the Least Common Multiple and Greatest Common Divisor of two or more numbers.
Open toolConvert length, weight and temperature units instantly.
Open toolCreate a Brazilian PIX copy-and-paste QR code in seconds.
Open tool