Count the number of items in the data set and arrange the data points in order from least to greatest. For example, if your data set is {3, 9, 2, 4, 6, 3, 2, 9, 10, 4, 4}, you have 11 data points and should order them {2, 2, 3, 3, 4, 4, 4, 6, 9, 9, 10}.
Add 1 to the number of points in the data set. In this example, add 1 to 11 to get 12.
Multiply the result by 1/4 to find the item in the data set that serves as the first quartile. In this example, multiply 12 by 1/4 to get 3, meaning the third smallest number equals the first quartile. In this example, the third number is 3, meaning the first quartile equals 3.
Multiply the result by 3/4 to find the item in the data set that serves as the third quartile. In this example, multiply 12 by 3/4 to get 9, meaning the ninth smallest number equals the third quartile. In this example, the ninth number is 9, meaning the third quartile equals 9.