If you don't know an item's drop chance, you can look it up on WoWHead.
Make sure you are viewing the correct expansion for the version of the game you are playing, as drop chances can differ between expansions.
Note: Enter the drop chance as a percentage (e.g., 5 for 5%).
Enter the drop chance of an item and the number of runs you have completed. The calculator will estimate the probability of seeing the item at least once.