Math
Rule of Three Calculator
Simple rule of three: solve proportions in seconds.
If A is to B, then C is to X
X (result)
100
X = (B × C) / A
How it works
The rule of three solves direct proportions. Given three values A, B, C, we find the fourth X such that A/B = C/X.
X = (B × C) / A
Practical examples
If 10 km cost $50, how much for 25 km?
X = (50 × 25) / 10 = $125.00
If 3 people drink 6 L, how much do 5 drink?
X = (6 × 5) / 3 = 10 L
Frequently asked questions
Does it handle inverse proportions?
This version is for direct proportions. For inverse, use X = (A × B) / C (more workers → less time, e.g.).