Simplify a ratio to its lowest terms, or solve a proportion A : B = C : D for the missing value, for recipes, scaling, maps and more.
What is a ratio?
A ratio compares two quantities. 2 : 3 means "for every 2 of the first, there are 3 of the second". Ratios are used for recipes, mixing paint or concrete, map scales, screen sizes and splitting money.
Solving a proportion
Two equal ratios form a proportion. Cross-multiply to find the missing value:
A : B = C : D → A × D = B × C
D = B × C ÷ A C = A × D ÷ B
Worked examples
- Scaling: 2 : 3 = C : 12. C = 2 × 12 ÷ 3 = 8.
- Recipe: a recipe uses 3 cups of flour for 4 servings. For 10 servings: 3 : 4 = C : 10, so C = 7.5 cups.
- Simplifying: 18 : 24 divides by 6 to give 3 : 4.
Ratios and percentages
A ratio of 2 : 3 means the first part is 2 out of 5 in total, so 40%. That is how a 2 : 3 split of $1,000 gives $400 and $600.
Frequently asked questions
Can a ratio include decimals?
Yes. To simplify, the calculator first scales decimals up to whole numbers, so 1.5 : 2.5 simplifies to 3 : 5.
What is the difference between a ratio and a fraction?
A fraction compares a part with the whole (2/5), while a ratio usually compares two parts (2 : 3).
Related tools
Results are estimates for planning and education only. They are not financial, tax, legal or medical advice. Check important numbers with a qualified professional.