Out of caster sugar?

The closest substitute for caster sugar is superfine sugar, also sold as baker's sugar in the US. If you only have regular white granulated sugar, briefly process it into smaller crystals and substitute it by weight.

What is caster sugar?

Caster sugar has finer crystals than ordinary granulated sugar. It dissolves more readily, which is useful in meringues and delicate cakes. It still has a grainy texture; it isn't powdered sugar. Domino's explanation of baker's sugar shows the same distinction between ultrafine crystals and powdered sugar.

How to make a substitute

1. Put white granulated sugar in a clean, completely dry food processor or blender. Use enough for the blades to catch it.
2. Pulse briefly, checking between pulses. Stop when the crystals are finer but still look like sugar, before they become powder.
3. Let the dust settle before opening the lid. Weigh out the amount your recipe calls for.

For a recipe asking for 150 g caster sugar, use 150 g of your finer sugar. King Arthur Baking recommends a substitution by weight, because crystal sizes pack differently in a measuring cup. If your recipe only gives cups, packaged superfine sugar is the simplest way to stay close to the intended ingredient.

Can I use regular granulated sugar directly?

Often, yes, in cakes and cookies. The tradeoff is the larger crystals: they may need longer to dissolve or cream into butter. For a meringue or macaron recipe written for caster sugar, using superfine sugar keeps that variable closer to the original method. Don't add extra liquid to dissolve the sugar unless the recipe tells you to.

Powdered sugar isn't a straight replacement. Its texture is different and many US brands contain starch. Brown sugar also changes the flavor, color, and moisture. Save those swaps for recipes designed around them.

Using it in this recipe box

The Earl Grey cake uses caster sugar in the cake batter. The popcorn macarons use caster sugar for the egg whites and icing sugar in the almond mixture: keep those two quantities separate.

Making macarons? Check the almond flour guide before you weigh the dry ingredients.