According to the Domino Sugar company, the amount of powdered sugar in a pound is 3.75 cups, although that measurement was taken with unsifted sugar. Because sifting fluffs up the sugar granules, it takes more unsifted confectioner's sugar than sifted sugar to make a pound.
Confectioner's sugar is made from regular granulated sugar that has been finely ground into powder. To prevent crystallization and lumping, 3 percent cornstarch is added to the sugar. A number followed by the letter "X" is sometimes displayed on the box or bag that confectioner's sugar comes in to correspond with the number of times the sugar was ground during the manufacturing process.