Do not mistake weight ounces with volume ounces when measuring powdered sugar. Know exactly how much you are purchasing. Everyone is aware that one cup equals eight ounces, right? In actuality, when measuring components for a recipe, 1 cup does not necessarily correspond to 8 ounces.
- We know that 1 cup of water weighs 8 ounces, but because honey is more viscous than water, 1 cup of honey weighs 12 ounces.
- Powdered sugar directly from the box or plastic bag weighs 4 1/2 ounces per cup; therefore, a 1-pound box (or 16 ounces) provides approximately 3 1/2 cups of powdered sugar.
- If a recipe asks for powdered sugar, 4 ounces of powdered sugar will equal 1 dry measuring cup.
Powdered sugar becomes lighter and fluffier when sifted, so it occupies more space in the cup. 
What size package is available for confectioner’s sugar?
All About Sugar in Baking | Ask Baker Bettie
As illustrated by @Jefromi, they are often sold in boxes weighing one pound and sixteen ounces in the U.S. Either 3 1/2 to 4 cups, or, more simply, half the quantity in the bag, should be measured out for a 2-pound bag.