A ratio shows the relationship between two numbers. In many cases, a ratio is useful for making comparisons, such as in comparing the number of managers in a company to the number of employees. In this case, the ratio can tell you the average number of employees a manager is responsible for. Calculating a ratio is often a simple task, but you must take care not to mix up the orders of the numbers.

Keep the order of the ratio straight in your mind. For example, if you are calculating a ratio of adults to children, make sure you don't confuse the order of the numbers. The ratio of adults to children is different from the ratio of children to adults.

Count the number of each group. If you have 40 adults and 30 children, 40 and 30 are the numbers you will use.

Place the first number in front of the second, separating the numbers with a “to” or a colon. For the example of adults to children, the ratio is “40 to 30” or equivalently “40:30.”

Simplify the ratio. Divide each side by the greatest common divisor, or the biggest number that can divide both the numbers in your ratio. For example, the greatest common divisor of 30 and 40 is 10. Dividing 40 and 30 by 10 gives you 4 and 3. Rewrite the ratio 40:30 as 4:3, or 4 adults for every 3 kids.
References
 Photo Credit John Foxx/Stockbyte/Getty Images