The average potato is comprised of approximately 80 percent water. The percentage of water contained within a given potato varies somewhat depending on the type of potato it is; waxy types typically contain more water than floury types.
Water is typically the most prevalent element within any fruits or vegetable. By way of context, the water content within a potato is substantially lower, relatively speaking, than other fruits and vegetables, many of which have water compositions over 90 percent. Aside from water, the remainder of the potato is primarily comprised of starch, a little bit of protein and trace amounts of fat.