Yes, it works
Best ratio: Swap 1:1 by weight — the sweetness is identical, only the grain size differs.
Caster and granulated sugar are the same thing — sucrose — just ground to different sizes, so you can swap between them freely with no change in sweetness. Caster is simply finer, which makes it the better choice for most baking.
Why the grain size matters
Finer caster sugar dissolves faster and more evenly, so it blends into batters smoothly and creams with butter better, giving a lighter, finer-textured bake. That’s why recipes for cakes and meringues usually specify caster. Granulated works fine in most things too, but for a delicate crumb, or anything where the sugar needs to dissolve quickly, caster has the edge. For a crunchy topping, though, coarser sugar is better.
When it works
- Almost any baking — cakes, biscuits, meringues — where finer sugar dissolves better.
- Creaming with butter for a smoother, better-aerated mix.
- Sweetening drinks and sauces where you want it to dissolve quickly.
When it doesn't work
- Nowhere, really — but for a crunchy sugar topping, coarser granulated or demerara gives more texture.
Taste & texture difference
There's no difference in taste or sweetness. Caster's finer grains just dissolve more quickly and evenly, which gives a smoother batter and a finer crumb.