Ratio Calculator
Enter A, B and C to find D so that A : B = C : D, and see the ratio in its simplest form.
Result
- A : B in simplest form
- 3 : 4
- A ÷ B as a decimal
- 0.75
- D = B × C ÷ A.
How to use Ratio Calculator
- Enter A and B, the ratio you know.
- Enter C, the number that matches A.
- Read D, the missing number, and the simplified ratio.
About this tool
A ratio compares two amounts. 3 : 4 means 3 parts to 4 parts. Two ratios are equal if they simplify to the same form, so 3 : 4 = 9 : 12.
To find the missing number in A : B = C : D, multiply B by C and divide by A. For 3 : 4 = 9 : D, D = 4 × 9 ÷ 3 = 12.
How it is calculated
D = B × C ÷ A.
Simplify by dividing A and B by their greatest common factor.
Worked examples
- 3 : 4 = 9 : 12
- 12 : 18 simplifies to 2 : 3.
- 5 : 2 = 20 : 8
Tips for better results
- Use the same units on both sides, such as both in cm.
- To scale a recipe, set A and B to the old and new amounts.
Frequently asked questions
How do you simplify a ratio?
Divide both numbers by their greatest common factor.
How do you solve a proportion?
Cross-multiply: for A : B = C : D, D = B × C ÷ A.
Are my numbers saved or sent anywhere?
No. The calculation runs in your browser on your own device. Nothing you type is sent to Paul Digital or any other server.
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Privacy
The calculation runs in your browser on your own device. The numbers you type are not sent to Paul Digital or any other server. Results are estimates from the numbers you enter and are not financial, tax or legal advice.