Rule of three calculator
Exact calculation2026 figuresFreeNo sign-up
is to
is toX
How it was calculated
Direct (the more, the more): X = B × C ÷ A. Example: 3 kg cost €6, how much do 5 kg cost? Inverse (the more, the less): X = A × B ÷ C. Example: 3 people do a job in 6 days, how long do 5 people take?
Frequently asked questions
How do you do a rule of three?
If A is to B, C is to X: X = B × C ÷ A.
When is the rule of three inverse?
When one quantity goes up and the other goes down, like more people doing the same job in fewer days.