In tableau, right-click on a measure and select percentile from the drop-down menu. You may adjust the percentile to any number between 0 and 1 by clicking Edit in Shelf (4th from the bottom). Tableau uses a two-step calculation to determine percentiles. Get the rank in the ordered set: (nth percentile(number of elements in set -1)) + 1. Then, using the rank from step 1 above, calculate the value of the element in the set that corresponds to the rank.