Ratio calculator
Leave one box empty to solve A : B = C : D, and see the ratio in its simplest form.
Result
How it works
A : B = C : D ⇔ A × D = B × CSo D = B × C ÷ A, C = A × D ÷ B, B = A × D ÷ C and A = B × C ÷ D.
Worked example
A 1920 × 1080 image scaled to 1280 wide:
- 1920 : 1080 = 1280 : D
- D = 1080 × 1280 ÷ 1920
- D = 720, and the ratio is 16 : 9
Frequently asked questions
How do I solve a proportion?
Cross-multiply. If A:B = C:D and D is missing, D = B × C ÷ A. Leave any one of the four boxes empty and the calculator finds it.
How do I simplify a ratio?
Divide both sides by their greatest common divisor: 1920:1080 ÷ 120 = 16:9. Decimals are scaled to whole numbers first.
Where are ratios used?
Screen and photo aspect ratios, recipes for a different number of servings, map scales, mixing ratios for concrete or paint, and splitting money in proportion.