Potatoes are heavier than they look, so account for half a pound of raw potatoes per serving when considering how much mashed potatoes you have to make. Bags of red potatoes are generally sold in 3lb. packages while russet potatoes are sold in 5 and 10 pound bags. Remember red potatoes won't be peeled so you don't lose the weight of the potatoes while russet potatoes will lose at least 5-10% of their weight in preparations.