Write down the numbers in numerical order that you are computing the interquartile range for. Visually observing the numbers makes the computation easier. In this example, we will use 1, 2, 3, 5, 7, 11, 15, 19 and 21.
Determine the median. It is the number in which there is an equal number of numbers in the set you are working with below it and above it. In our example, the median is 7. It has four numbers above and four below it in the set.
Determine the first and third quartiles. The first quartile is the subset of numbers below the median. In our example, it is 1, 2, 3 and 5. The third quartile is the subset of numbers above the median. In this example, it is 11, 15, 19 and 21.
Compute the median for the first and third quartiles. This is done in the same way as Step 2, but because there are four numbers in each subset it is the number in the middle of the two numbers in the middle of the series. For quartile one, this would be 2.5 as that is between 2 and 3. For quartile three it is 17, as it is in between 15 and 19.
Compute the interquartile range. To do this, you subtract the mean of the first quartile from the mean of the third quartile. In this example that means subtracting 2.5 from 17, resulting an interquartile range of 14.5.