500 grams of powdered sugar to cups

Powdered sugar fights you in ways granulated never does. The grains are pulverized to roughly 10X fineness (the "10X" on the bag refers to how many times it has been milled), and the resulting particles are so small they behave more like a powder of flour than a crystalline sugar. That fineness creates two problems for measurement. First, the particles compact under their own weight in the bag, so the top of a fresh bag is fluffier than the bottom. Second, manufacturers blend in roughly 3 percent cornstarch as an anti-caking agent, which means you are measuring a sugar-starch composite, not pure sucrose. Add the fact that the powder is hygroscopic and will absorb moisture from a humid kitchen within minutes of opening the bag, clumping into pockets that resist a measuring cup, and you have an ingredient that can swing 20 to 30 percent by weight depending on how it was scooped, sifted, and stored. The King Arthur Baking ingredient weight chart pegs unsifted confectioners' sugar at 113 grams per US cup, which is the value KitchenMath uses as the default, but that number assumes a freshly opened bag, gently spooned, not packed.

The single most common mistake is treating powdered sugar like granulated and dipping the measuring cup straight into the bag. With granulated sugar that scoop-and-level method works because the crystals are uniform and free-flowing, so the cup fills consistently. With powdered sugar, the dip compresses the powder against the bottom of the bag and packs the cup. A single packed cup can weigh 140 grams or more, which is roughly 25 percent over the King Arthur reference of 113 grams. In a royal icing, that extra sugar turns a pipeable consistency into a dry crumble. In a glaze, it shifts the ratio so far that no amount of added milk recovers a smooth pour.

The correct technique is to fluff the sugar in the bag with a fork or whisk first, breaking up any clumps that have formed since the bag was opened. Spoon the loosened sugar lightly into the measuring cup until it mounds above the rim, then sweep the excess off level with the straight back of a knife. Do not tap the cup, do not press the powder, and do not shake it. If the recipe calls for sifted powdered sugar, sift first and then spoon into the cup, because sifted volume runs about 15 grams lighter per cup than the unsifted King Arthur reference. The single best move, on any recipe that calls for more than half a cup, is to skip the cup entirely and weigh: 113 grams per cup, every time, regardless of humidity or how long the bag has been open.

Density precision matters most in the recipes built almost entirely from powdered sugar, where the ingredient is the structure rather than a supporting note. American buttercream lives or dies on the sugar-to-butter ratio; an extra 25 grams per cup multiplied across four cups of sugar is 100 grams of unintended sugar, enough to render the frosting grainy and stiff. Royal icing is even less forgiving because the consistency is calibrated to a specific water-to-sugar ratio for piping versus flooding, and a packed measure pushes flood icing into a state that will not self-level. Glazes for donuts, scones, and bundt cakes show the error on the surface as either a watery drip or an opaque paste. By contrast, a recipe that uses powdered sugar only for dusting or a tablespoon-scale addition to whipped cream is far more tolerant.

Substitutions are tricky because the cornstarch is functional, not decorative. Granulated sugar pulverized in a blender or spice grinder gives you the particle size but not the anti-caking behavior, and it will clump faster in icings and weep moisture in meringues. If you must improvise, blend 1 cup of granulated sugar with 1 tablespoon of cornstarch on high until powdery (about 60 seconds), then let it settle in the blender jar before opening the lid. The result works for dusting and for cooked frostings; it is less reliable in raw applications like royal icing where the cornstarch ratio drives the set.