Rule of Three Calculator
Enter three values in a direct proportion to calculate the fourth, and compare two values as a simplified ratio, decimal, and percent.
Runs locally in your browser. This tool does not upload your input.
Rule of three
A is to B as C is to X.
Proportion comparison
Results
- Missing value (X)
- 20
- Ratio
- 16:9
- Decimal
- 1.778
- Percent
- 177.78%
Values calculated locally.
How to use the rule of three calculator
Find the fourth value in a direct proportion, or compare two values to see their ratio, decimal, and percentage.
When to use it
Use this calculator when three proportional values are known and you need the missing one, such as price versus quantity or distance versus time.
Steps
- Enter the base pair of values.
- Enter the known value in the second pair.
- Read the calculated missing value and copy it if needed.
Formula
- Direct proportion
X = (B x C) / A
Local calculation
All values are calculated in your browser and are not sent to ProntoUso.
Rule of three FAQ
- Does it handle inverse proportion?
- This first version is focused on direct proportion, the most common everyday case.
- Can I compare two numbers without the rule of three?
- Yes. The proportion section shows ratio, decimal value, and percentage.
- Can I use decimal values?
- Yes. Decimal values are supported as long as they form a valid calculation.