⚖️ Proportion Calculator
Solve ratio and proportion equations instantly. Leave exactly one field blank to calculate its value.
The Comprehensive Guide to Proportion Calculator
What is a Proportion Calculator?
The Proportion Calculator (often called the rule of three calculator) solves for an unknown value in a mathematical proportion equation. By giving the calculator any three known values, it uses cross-multiplication to instantly find the missing fourth variable.
The Mathematical Formula
Solving for a missing value in equal ratios.
Calculation Example
Let's solve for D if A = 2, B = 4, and C = 3:
- Set up the ratio: 2 / 4 = 3 / D
- Cross multiply: 2 × D = 4 × 3
- Simplify: 2D = 12
- Solve for D: D = 12 / 2 = 6
Strategic Use Cases
- Dosing & Chemistry: Very common in nursing to calculate correct medication doses based on patient weight and known drug concentrations.
- Baking & Cooking: Scaling a recipe up or down. If a recipe for 4 people requires 3 cups of flour, how much flour is needed for 10 people?
- Currency & Exchange: Converting money when you know the exchange rate between two specific currencies.
Frequently Asked Questions
What is the 'Rule of Three'?
The 'Rule of Three' is a widely used short-hand name for solving direct proportions. If you know three numbers that are proportional to each other, you can always find the fourth using this exact mathematical rule.
Can this solve inverse proportions?
No, this specific calculator formula (A/B = C/D) only solves for direct proportions (when one side increases, the other increases). Inverse proportions require a different formula (A × B = C × D).
How accurate is this calculator?
Our calculator uses industry-standard formulas to provide the most accurate results possible. However, it should be used for informational purposes only and not as a basis for formal calculations or legal advice.