How to use Proportion Calculator
- Choose which value (A, B, C or D) is missing.
- Enter the three known values.
- Read the missing value and the completed proportion.
About this tool
A proportion says that two ratios are equal: A ÷ B = C ÷ D. Multiplying across gives A × D = B × C, so the missing value is the product of the other diagonal pair divided by the remaining number. This is also called the rule of three.
Example: if 3 notebooks cost 12, how much do 9 cost? Write 3 : 12 = 9 : D and the answer is D = 12 × 9 ÷ 3 = 36.
Privacy: This tool runs in your browser. What you type or choose is not sent to a server by this website.
Common questions
When does it not work?
The number you divide by cannot be 0. The tool tells you which value is the problem.
Does it work for inverse proportions?
No. For inverse proportion (more workers, less time) the products stay equal instead of the ratios. Multiply the known pair and divide by the third number yourself.