Caster sugar is the British name for superfine sugar — ordinary white granulated sugar milled to a finer grain so it dissolves quickly. It is not powdered sugar. In the US, look for superfine or bar sugar, or make your own by pulsing granulated sugar in a blender for 30 seconds.
Caster sugar turns up constantly in British baking books and almost never on an American shelf, which sends a lot of people to the store looking for something that is sold there under a different name. It is the same sugar you already have, just ground smaller. The distinction matters in a handful of recipes and matters not at all in most of the others, and knowing which is which saves you a trip.
| Name | Also called | Grain | Use it for |
|---|---|---|---|
| Granulated | White, table sugar | Medium | Everyday baking, dissolving in liquid |
| Caster | Superfine, bar sugar | Fine | Meringues, creamed cakes, shortbread |
| Powdered | Confectioners', icing | Dust, plus cornstarch | Frosting, dusting - not a caster substitute |
| Sanding | Pearl, decorating | Coarse | Sprinkling on top, does not melt |
It is granulated sugar, only finer
Caster sugar and granulated sugar are chemically identical — both are pure sucrose from cane or beet. The only difference is crystal size. Granulated is about 0.5 mm across; caster is roughly half that. Smaller crystals have more surface area, so they dissolve faster and beat more air into butter, which is why British recipes call for it in cakes and meringues.
It is not powdered sugar
This is the mistake that ruins things. Powdered sugar, also sold as confectioners' or icing sugar, is ground to a flour-like dust and cut with about 3% cornstarch to stop it clumping. Substituting it for caster sugar gives you a dense, oddly soft result and a faint chalky taste from the starch. If a recipe says caster, it wants a granular sugar you can still feel between your fingers.
Making your own takes 30 seconds
Put granulated sugar in a dry blender or food processor and run it for 20 to 30 seconds until the grains look like fine sand but not dust. Let it settle for a minute before opening the lid, or you will lose a cloud of it. Measure it after grinding rather than before: the finer grains pack tighter, so a cup of homemade caster weighs a little more than a cup of the granulated you started with.
When the swap actually matters
It matters for meringues, where undissolved crystals weep syrup out of the finished shell, and for delicate creamed cakes and shortbread, where the finer grain gives a tighter crumb. It does not matter for anything that gets dissolved in liquid, anything with a long bake, or most cookies — use granulated straight across and nobody will know.
Questions we get asked
What is caster sugar in American terms?
Superfine sugar, sometimes sold as bar sugar or baker's sugar. It is the same white granulated sugar ground to a finer crystal so it dissolves faster.
Can I use granulated sugar instead of caster sugar?
In most recipes, yes, measure for measure. The exceptions are meringues and delicate creamed cakes, where the larger crystals may not dissolve fully and can leave a gritty texture or weep syrup. For those, grind the sugar finer first.
Is caster sugar the same as powdered sugar?
No. Powdered sugar is ground to a dust and contains about 3% cornstarch to keep it free-flowing. Swapping it in for caster sugar gives a dense, soft, faintly chalky result. Caster is still granular.
How do you make caster sugar at home?
Pulse granulated sugar in a dry blender or food processor for 20 to 30 seconds, until it looks like fine sand rather than dust. Let it settle before opening the lid, and measure it after grinding, not before.
Does caster sugar weigh the same as granulated?
By weight they are identical, which is why gram measures need no adjustment at all. By volume the finer grains pack a little tighter, so a cup of caster weighs slightly more than a cup of granulated - one more reason to use a scale for baking.
Why do British recipes call for caster sugar?
It is simply the standard baking sugar sold in the UK, in the way granulated is the default in the US. British creaming methods also lean on the finer grain to beat more air into butter.
Where do I buy superfine sugar?
Most large supermarkets carry it in the baking aisle, often labeled superfine or baker's sugar, and bar sugar sits with cocktail supplies. It costs noticeably more than granulated, which is a good argument for the blender.