Ratio Calculator
Solve proportions and simplify ratios. Free, fast and easy to use.
How it works
Fill in A : B = C : D and the calculator finds the missing value D. It also reduces the ratio A : B to its simplest form, which is useful for scaling recipes, maps, drawings and mixtures.
Formula
A : B = C : D → D = B × C / A
Worked examples
- A: 16 · B: 9 · C: 1920 → Missing value D: 1,080
- A: 3 · B: 4 · C: 12 → Missing value D: 16
- A: 1 · B: 3 · C: 2.5 → Missing value D: 7.5
Calculations run locally in your browser. Results are for information only — for engineering, electrical or financial decisions, verify the assumptions and applicable standards.
Frequently asked questions
How do I solve a proportion?
Cross-multiply: in A : B = C : D, the missing value is D = B × C / A. For a 16 : 9 screen 1920 pixels wide, the height is 9 × 1920 / 16 = 1080 pixels.
How do I simplify a ratio?
Divide both numbers by their greatest common divisor. 12 : 18 has a common divisor of 6, so it simplifies to 2 : 3.