The third quartile (also called upper quartile, most often denoted by Q3), is the value that marks three quarters (75%) of data points when they are sorted in increasing order. In other words, three-quarters of the data points are less than the upper quartile, and one-quarter are greater than the upper quartile. We can also say that the third quartile splits off the highest 25% of data points from the lowest 75% of data points.

To compute the third quartile:

1. Sort your data in increasing order.

2. Determine the upper half of your data:

   • Even number of observations: Split your data set in half and take the upper half.

   • Odd number of observations: Take all the numbers bigger than the number in the middle of your set.

3. Compute the upper quartile as the median of the upper half:

   • Odd number of observations: Take the number right in the middle.

   • Even number of observations: Take the average of the two values in the middle.

The third quartile is the right side of the box (if horizontal) or the top side (if vertical). The left/bottom side corresponds to the first quartile. Hence, the box's width is exactly the interquartile range (IQR).