How Do You Calculate Percentile?

Arrange all of the numbers in the data set into sequence from lowest to highest. This makes it easier to determine precisely which number in the set is at a specific percentile point.

Count the number of items in the data set, and multiply the result by the percentile desired. For example, the 90th percentile of a data set of 100 numbers would require multiplying 0.90 by 100. The result is the position of the number in the data set that is in the 90th percentile place. Anything below that number is a data set containing the lowest 90 percent of the numbers in the set, while anything higher contains the highest 10 percent of the set of numbers. If the placement number is not a whole number, then round up to the nearest whole number, and use that instead.

Move from left to right in the data set, from lowest to highest number, until reaching the location of the percentile place. In the case of 90th percentile out of 100 numbers, this would require counting from the left until reaching the 90th number. The number reached is the 90th percentile placement.