How to use this calculator
- Choose which term is unknown.
- Enter the other three values.
- The missing value, the simplified ratio A : B and its decimal value are shown.
Formula
In a proportion, the cross products are equal, so any one term can be found from the other three.
A : B = C : D ⇔ A × D = B × C
D = B × C / A
Example
- 3 : 4 = 9 : D.
- D = 4 × 9 / 3 = 12.
- Resize a 1920 × 1080 image to 1280 wide: height = 1080 × 1280 / 1920 = 720.
Frequently asked questions
How do I simplify a ratio?
Divide both terms by their greatest common factor. 18 : 24 → GCF 6 → 3 : 4. Decimal ratios are first scaled to whole numbers.
What's the difference between a ratio and a fraction?
A fraction compares a part to a whole; a ratio can compare any two quantities, including part to part (e.g. 2 boys : 3 girls).
How do I scale a recipe?
Set A : B as the original servings to an ingredient amount, enter the new servings as C, and solve for D.