Updated: July 2026
What is it
The Rule of Three Calculator is an essential mathematical tool that helps you find a fourth unknown value when you know the proportion between three other numbers. It is perfect for solving everyday proportion problems, from adapting cooking recipes to calculating fuel consumption for a trip.
Our tool works online and solves both the Direct and Inverse rule of three, in addition to showing the mathematical step by step of how the calculation was made so you can learn the formula!
How to use the Rule of Three
1. Straight Rule of Three
We use the Straight Rule of Three when two quantities increase or decrease by the same proportion. If one goes up, the other goes up. If one falls, the other falls.
Example: If 2kg of meat costs R$60, how much will 5kg cost?
- A = 2
- B = 60
- C = 5 -X = ? (Result: R$150)
Direct Formula: We cross multiply. X = (B × C) ÷ A
2. Inverse Rule of Three
We use the Inverse Rule of Three when quantities are opposite: if one increases, the other decreases proportionally.
Example: If 4 bricklayers build a wall in 10 days, in how many days will 8 bricklayers build the same wall? (More bricklayers = Fewer days).
- A = 4
- B = 10
- C = 8 -X = ? (Result: 5 days)
Inverse Formula: We multiply in a straight line. X = (A × B) ÷ C
Tips
- The biggest mistake in tests and competitions is applying the direct rule to problems with inverse quantities (such as Speed x Time). Always ask yourself: "if I increase the value on the left side, will the value on the right side increase or decrease?". If the answer is decreasing, select the Inverse Rule of Three tab!
- Enter your numbers into the calculator and follow the explanation box to memorize the difference between "cross multiplication" and "straight multiplication".