How to calculate the interquartile range.

To find the interquartile range you need to find the upper and lower quartiles. To do this find the median of your data set. Next take the numbers above the median, and find the median of those numbers, that is your upper quartile. Take the numbers below the median and find the median of those numbers, that is your lower quartile. Subtract the lower quartile from the upper quartile, and that is your interquartile range. 

In a box plot, there is a middle (which is the median of the data set), and the upper and lower quartile ranges.

For example, if your data set was:
1,4,7,10,14,17,18

10 would be your median because it is in the middle.

The lower quartile range would be set out like this:
1,(4),7
4 is your lower quartile because it is in the middle of the lower quartile.

The upper quartile range would be set out like this:
14,(17),18
17 is your upper quartile because it is in the middle of the upper quartile.

To find the interquartile range, you do 17-4, because those are your lower and upper quartile medians. This would get you 13, which in this case would be your interquartile range.