Rule of Three Calculator (Proportions)
Rule of three calculator: solve a direct proportion A/B = C/X for any missing value.
AB = CX
How to use this calculator
- Choose the quantity you want under “Solve for”.
- Type the known values and pick their units: any combination works.
- The result updates as you type. Open “Show the steps” to see the substitution.
If A corresponds to B, the rule of three finds what C corresponds to: X = B × C ÷ A. It is the everyday tool for scaling recipes, prices and quantities.
This is a direct proportion: when one value doubles, the other doubles too.
Formula
AB = CX
| Symbol | Quantity | Units |
|---|---|---|
| A | A | — |
| B | B (corresponds to A) | — |
| C | C | — |
| X | X (corresponds to C) | — |
Worked example
Given: A = 3 · B = 4.5 · C = 7
X = B · CA
Result: X = 10.5
Frequently asked questions
If 3 kg cost $4.50, how much do 7 kg cost?
X = 4.50 × 7 ÷ 3 = $10.50.
What is an inverse rule of three?
When one value goes up and the other down (more workers, fewer days), use X = A × B ÷ C instead.