How to calculate ratio
A : B = C : D → D = B × C ÷ A
Worked example. 3 : 4 = 15 : ?: 20.
Example inputs — A: 3; B: 4; C: 15. Starting values are illustrative.
Simplified ratio 3 : 4
From George Guju, founder of Calculyn · Educational calculator guidance, not professional advice.
How it works
Two ratios are in proportion when their cross products are equal. Solve for the unknown by cross-multiplying.
Assumptions & limitations
- Results are rounded for display.
Formula and assumptions reviewed October 2026 by the Calculyn team. How we build our calculators
Frequently asked questions
How do I scale a recipe?
Set A to the original servings, B to an ingredient amount, and C to the servings you want.