Potatoes are not nearly as solid as they seem to be. A potato is made up of roughly 80 percent water and only 20 percent solids. Of the 20 percent solids, almost all of it is starch. All of that water really reduces the number of calories found in the carb, as an 8-ounce potato contains only 100 calories. Since they are made up of such a high percentage of water, no wonder potatoes require so much water for growing. 1 pound of potatoes requires roughly 34 gallons of water to grow.