Rule of Three Calculator
Solve a direct proportion: if A gives B, what does C give?
Results
X
15
What this tool does
The rule of three is the workhorse of practical arithmetic: scaling a recipe, converting a price per unit, working out how long a job takes with more people. It assumes the two quantities are directly proportional, so doubling one doubles the other.
Formula
x = b × c ÷ a
Variables
| Symbol | Meaning | Unit |
|---|---|---|
a | A | — |
b | B | — |
c | C | — |
x | X | — |
Worked example
- A4
- B10
- C6
- X15
Limitations
- The calculation runs at full precision and only the display is rounded. If you copy an intermediate value and retype it, small differences can appear.