Ratio Calculator (Proportion Solver)

How to use this calculator

The calculator solves A : B = C : D. Enter any three values and the fourth is found. To change which value is calculated, press × on it.

The formula

A ÷ B = C ÷ D → A × D = B × C

Worked examples

Scaling a recipe. A recipe uses 3 cups of flour for 4 servings. How much for 20 servings? 3 : 4 = C : 20 → C = 3 × 20 ÷ 4 = 15 cups

Map scale. On a 1 : 50,000 map, 6 cm on paper is 1 : 50,000 = 6 : D → D = 300,000 cm = 3 km.

Mixing. Concrete at 1 : 2 cement to sand needs 2 × 50 = 100 kg of sand for a 50 kg bag of cement.

Direct vs inverse proportion

This calculator is for direct proportion (both values grow together). For inverse proportion — such as more workers finishing a job in fewer days — multiply instead: 4 workers × 10 days = 5 workers × 8 days.

Frequently asked questions

How do I solve a proportion?

Cross-multiply. If A:B = C:D, then A × D = B × C. For 3:4 = C:20, C = 3 × 20 ÷ 4 = 15.

What is the "rule of three"?

The same idea — if 3 kg costs 4, then 15 kg costs 20. Three known values give the fourth.

How do I simplify a ratio?

Divide both parts by their greatest common factor. 15:20 simplifies to 3:4.

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