How Many Grams in a Cup of Powdered Sugar? 120, With an Asterisk

The question how many grams in a cup of powdered sugar has a one-number answer and a footnote that matters more than the number. The answer is 120 grams, for one US cup, spooned into the cup and leveled, unsifted. The footnote is that powdered sugar is the most compressible ingredient in the baking aisle, and the way you fill the cup can swing that number by a third. Frosting is forgiving. Meringue is not. The method decides which one you are making.

The 120-gram cup, and when it stops being 120 grams

Most recipes that call for powdered sugar mean it unsifted, measured straight from the bag with a spoon-and-level technique. Sifting usually happens after measuring, to knock out lumps before the sugar hits the butter. That convention is why 120 grams is the working number across conversion charts. But watch what happens when the method changes:

The sifted column runs about 15 to 20 percent lighter because sifting whips air through the powder. Scooping straight from the bag with the measuring cup does the opposite: it compresses the sugar and can pack in roughly 30 percent more than the recipe intends. That is the difference between silky buttercream and frosting so sweet it makes your teeth ache. If a recipe says "sifted powdered sugar," it means sift first, then measure, and you want the lighter column.

The cornstarch footnote

Here is the tangent worth taking, because it explains a substitution mistake I see constantly. Powdered sugar is not just finely ground sugar. It contains about 3 percent cornstarch by weight, added to keep it from caking into a brick in the box. That cornstarch is harmless in frosting and dusting, but it changes how the sugar behaves in batters and doughs. You cannot swap powdered sugar for granulated sugar by weight in a cake and expect the same crumb, because you are also adding starch that was never in the recipe.

The cornstarch also explains a regional curiosity. In Australia, "pure icing sugar" contains no anti-caking agent at all, and it sets noticeably harder, which is why it is specified for decorative work like extension piping on wedding cakes. American powdered sugar, cornstarch included, stays softer. Same name family, different product, different result.

And if you run out mid-recipe, the fix is a blender: 1 cup (200g) of granulated sugar plus 1 tablespoon of cornstarch, blended until fine and powdery, gives you about 1 cup, or 120g, of powdered sugar. It will be slightly less silky than the commercial 10X grind. In frosting, nobody will know the difference.

The buttercream math

A classic American buttercream runs 3 to 4 cups of powdered sugar per stick of butter, which is 360 to 480 grams of sugar against 113 grams of butter. That ratio is why a single 1-pound box, 454 grams, covers about one layer cake or a dozen cupcakes with tall swirls. Need grand swirls on 24 cupcakes? Plan on two boxes. Mixing a thin glaze for cinnamon rolls? One box stretches much further, because the liquid ratio does the work instead of the sugar.

So the next time a recipe says "2 cups powdered sugar," ask the question the recipe did not answer: whose cup? Sifted or scooped, spooned or packed? The 120-gram answer is right for the standard method, but the recipe will not tell you which method it assumed, and that silence is where lumpy frosting comes from. When in doubt, weigh 120 grams per cup and sift after measuring. The scale settles the argument before it starts.

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