A 1 lb box of powdered sugar holds about 4 cups. The arithmetic is simple: 1 lb is 454 g, and one cup of powdered sugar weighs 113 g spooned and levelled, so 454 divided by 113 is 4 cups. Powdered sugar — also called confectioners sugar or icing sugar — is granulated sugar ground to a fine powder, with a little cornstarch added to stop it clumping. It is sold in a 1 lb box because most bakers use it for frosting, dusting and glazes, and go through it more slowly than granulated sugar. Four cups is about four batches of buttercream at 1 cup each, or eight dustings of a cake at half a cup each.
The cup weight of powdered sugar depends on whether it is sifted. Unsifted, spooned and levelled, a cup weighs 113 g, and a 1 lb box holds 4 cups. Sifted, a cup weighs about 100 g, because sifting adds air and makes the sugar lighter — and a 1 lb box of sifted sugar would hold about 4.5 cups. The difference matters in frosting: a buttercream made with sifted sugar will be lighter and smoother, but if you measure the sugar after sifting, you will use less sugar than the recipe expects, and the frosting will be too soft. Measure first, then sift — that is the standard practice, and it keeps the measurement consistent.
Box of powdered sugar conversion table
The table below shows how many cups you get from fractions of a 1 lb box, and how many cups of powdered sugar weigh how many grams. All figures use 113 g per cup, unsifted, spooned and levelled.
| Amount | Cups (approx.) | Weight |
|---|---|---|
| 1/4 bag | 1.00 cups | 114 g |
| 1/2 bag | 2.00 cups | 227 g |
| 3/4 bag | 3.00 cups | 340 g |
| Full bag | 4.00 cups | 454 g |
| Cups | Grams (approx.) |
|---|---|
| 1 cup | 113 g |
| 2 cups | 226 g |
| 3 cups | 339 g |
| 4 cups | 452 g |
| 5 cups | 565 g |
| 10 cups | 1,130 g |
King Arthur lists 113 g per cup unsifted; sifted powdered sugar is about 100 g per cup because sifting adds air. Measure first, then sift, to keep the measurement consistent.
Worked examples
Buttercream for a birthday cake
A buttercream for a birthday cake uses 2 cups of powdered sugar — half a 1 lb box — beaten with 1 cup of butter, a splash of milk and a teaspoon of vanilla. One box holds 4 cups, so it covers two batches of buttercream with enough left over for a dusting. Sift the sugar before adding it to the butter, so the frosting is smooth and free of lumps — but measure it first, then sift, so you use the full 2 cups. Beat the butter until pale and fluffy, add the sugar in two parts, and add the milk a tablespoon at a time until the frosting is spreadable.
A dusting for a sponge cake
A dusting for a sponge cake uses about 1/4 cup of powdered sugar, sifted over the top through a fine sieve. One 1 lb box holds 4 cups, so it covers sixteen dustings — years of cake dusting for most households. Sift the sugar from a height so it falls evenly, and dust the cake just before serving, because the sugar absorbs moisture from the cake and disappears if you dust it too early. A paper doily or a piece of lace laid over the cake before dusting makes a pretty pattern.
A glaze for a batch of doughnuts
A glaze for a batch of twelve doughnuts uses 1 cup of powdered sugar — a quarter of a 1 lb box — whisked with 2 tablespoons of milk and a teaspoon of vanilla. One box holds 4 cups, so it covers four batches of doughnuts. The glaze should be thick enough to coat the doughnuts but thin enough to drip — add the milk a teaspoon at a time until the consistency is right. Dip the warm doughnuts in the glaze and let them set on a rack. If the glaze is too thin, add more sugar; if it is too thick, add more milk a teaspoon at a time.
How the figure is worked out
The figure above is worked out by simple division, not by guessing. The package weight is a standard 1 lb (454 g) box of powdered sugar, unsifted. Divide that weight by the grams in one cup and you get the number of cups in the package. Because cup weights for dry ingredients depend on how the cup is filled, the result is approximate, and the honest way to use it is as a planning figure: it tells you how many batches a bag will cover, not the exact cup count to the last spoonful. The cup weight used here is 113 g per cup, spooned and levelled, sourced from King Arthur Baking. King Arthur lists 113 g unsifted; sifted powdered sugar is about 100 g per cup because sifting adds air.
Practical guidance
Powdered sugar clumps easily, and that is the most common problem with it. The fine powder absorbs moisture from the air, and a box left open in a humid kitchen will form hard lumps that do not dissolve. Store powdered sugar in its box in a cool, dry place, and seal the box tightly after each use. If the sugar has clumped, sift it before using — sifting breaks up the lumps and gives you a smooth powder. For frosting, always sift the sugar: unsifted powdered sugar makes a lumpy frosting, and no amount of beating will fix it.
Measure powdered sugar by weight when it matters. A cup weighs 113 g unsifted, but the exact weight depends on how you fill the cup and whether the sugar is sifted — sifted sugar weighs about 100 g per cup. Weighing removes the variable: 2 cups is 226 g, every time. In a buttercream, the sugar amount matters as much as the butter amount: too little sugar and the frosting is too soft and buttery; too much sugar and it is stiff and gritty. Weigh the sugar, sift it, and the frosting will be consistent every time. And remember that powdered sugar is not the same as granulated sugar — do not substitute them measure for measure, because powdered sugar is much lighter per cup and contains cornstarch.
Frequently asked questions
Is powdered sugar the same as icing sugar?
Yes. Powdered sugar, confectioners sugar and icing sugar are the same product — granulated sugar ground to a fine powder with a little cornstarch added to stop it clumping. The cup weight is the same: 113 g per cup unsifted. Buy whichever name is used where you live.
Why is sifted powdered sugar lighter per cup?
Because sifting adds air. Unsifted powdered sugar weighs 113 g per cup; sifted, it weighs about 100 g per cup. The difference is the air worked into the powder by sifting. Measure the sugar first, then sift it, so you use the full amount the recipe expects.
How many cups are in a 2 lb box of powdered sugar?
About 8 cups unsifted. A 2 lb box is 907 g, and at 113 g per cup that divides out to 8.03 cups. That is about four batches of buttercream at 2 cups each, or thirty-two cake dustings at a quarter cup each.
Can I make powdered sugar at home?
Yes. Blitz granulated sugar in a blender or food processor until it is a fine powder, then add a teaspoon of cornstarch per cup and blitz again briefly. Homemade powdered sugar works well for dusting and glazes, but it is rarely as fine as the bought kind, so it is less good for a smooth buttercream.
How long does powdered sugar keep?
Indefinitely, in a cool, dry place, sealed tightly. Powdered sugar does not go off, but it absorbs moisture and clumps. If it has clumped, sift it before using. Keep the box sealed and dry, and a box of powdered sugar will last for years.