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Ratio Calculator
Solve ratio and proportion problems by finding the missing value in A : B = C : X.
About the Ratio Calculator
Solve ratio and proportion problems by finding the missing value in A : B = C : X.
How to use the Ratio Calculator
- Enter A, B and C. The missing value X is calculated for you.
- Press Calculate Ratio to see X and the simplified ratio.
Formula and method
A ÷ B = C ÷ X, so X = (B × C) ÷ A (cross-multiplication).
Worked example
If 3 : 4 = 9 : X, then X = (4 × 9) ÷ 3 = 12.
Sharing in a ratio. To divide 50 in the ratio 3:2, there are 5 parts in total, so each part is 10. The shares are 30 and 20.
When to use the Ratio Calculator
- Splitting an amount in a given ratio, such as sharing costs or ingredients.
- Scaling a recipe or a mixture up or down.
- Simplifying a ratio to its lowest terms.
Common mistakes to avoid
- Adding the parts incorrectly when sharing a total. You divide by the sum of the parts, not by one part.
- Mixing units in the same ratio, such as minutes to hours.
Tips
- Ratios can be scaled up or down by multiplying or dividing both parts by the same number.
- Use ratios to scale recipes, maps and mixtures.
Frequently asked questions
How do I simplify a ratio?
Divide both parts by their greatest common factor. 12 : 18 simplifies to 2 : 3.
How do I simplify a ratio?
Divide all the numbers by their greatest common factor. 12:8 becomes 3:2.
What is the difference between a ratio and a fraction?
A ratio compares parts with each other (3:2). A fraction compares a part to the whole (3/5).