Caster Sugar

One US cup of caster sugar weighs 190 grams. Going the other way, 100 grams is about 0.53 cups.

Also called castor sugar, superfine sugar, baker's special sugar, bar sugar, fine sugar.

TL;DR: Caster sugar (superfine sugar, castor sugar) weighs 190 grams per cup based on King Arthur's chart entry for Baker's Special Sugar and USDA branded label data for retail superfine sugar. Roughly 5 percent lighter per cup than standard granulated sugar despite being chemically identical — the finer crystals trap slightly more air when spoon-and-leveled into a cup.

Caster sugar is finely-milled granulated sucrose, ground to a particle size intermediate between standard granulated (crystals roughly 0.5 mm) and powdered/confectioners' sugar (crystals roughly 0.05 mm and ground with cornstarch as an anti-caking agent). It goes by 'superfine sugar' and 'bar sugar' in the US, 'castor sugar' or 'caster sugar' in the UK and Commonwealth, and 'Baker's Special Sugar' in King Arthur's product line. The primary use case is applications where dissolution speed matters more than crystal texture — meringues, chiffon cakes, cold-mixed cocktails, syrups where the sugar needs to melt into the liquid quickly without a gritty aftertaste. In baked applications where the sugar dissolves during the bake regardless, caster and granulated sugar can be substituted for each other 1:1 by weight.

King Arthur Baking lists caster sugar (as Baker's Special Sugar) at 190 grams per cup on their ingredient weight chart. USDA Branded Foods carries an entry for Quick Dissolve Superfine Sugar (UPC 015800030850) at 203 grams per metric cup, which converts to approximately 192 grams per US cup — a one percent difference from KA that is within the noise of any measurement convention. This is a genuine convergence between publisher and producer data.

The interesting question is why caster sugar comes out lighter per cup than standard granulated sugar (KA lists granulated at 198 g/cup, USDA at 200 g/US cup). The molecules are identical sucrose crystals differing only in size, and the theoretical argument runs that finer particles should pack more tightly, giving a higher density per cup. Several online references treat caster and granulated as identical at 200 g/cup on this reasoning. But the measured values disagree with the theoretical prediction, and the physical explanation is straightforward: when very fine crystals are spooned into a measuring cup (rather than shaken down or tapped to settle), they behave more like a flour than like a coarse sugar, trapping air pockets between particles. A cup of caster sugar gently spooned in weighs less than a cup of granulated because the fine particles cannot settle out of the interstitial spaces the way larger crystals do. Under packed or tapped conditions, caster does become denser than granulated, but the spoon-and-level convention that recipes assume gives the 190-192 g/cup figure.

European and Commonwealth references sometimes list caster sugar at 225 grams per cup — a figure derived from the metric cup (250 mL) rather than the US cup (236.588 mL). Converted to US cup, that becomes 213 grams per cup and likely reflects a more packed measurement convention common in Continental baking. US recipes calling for caster sugar can safely use 190 g/cup; European recipes may assume a heavier cup, and weighing removes the ambiguity.

Practical baker recommendation: use 190 g/cup for caster sugar. If a recipe was developed with the 'same as granulated' 200 g/cup convention and you weigh out 190 g for a cup call, you'll be slightly under — enough to matter in a delicate meringue or Italian buttercream, negligible in a butter cake. When in doubt, weigh. If you don't have caster sugar and need it for meringues or cold-mixed drinks, blitzing granulated sugar in a food processor for 10 to 15 seconds produces a serviceable substitute; use the caster-sugar weight, not the granulated weight, in the recipe.