Caster sugar is ordinary white sugar milled to a finer crystal — around 0.3mm across instead of the 0.5mm of standard granulated — and if you are baking in the US or Canada, the closest caster sugar substitute is superfine sugar (sold as baker's sugar or berry sugar depending on the brand), or plain granulated sugar pulsed in a food processor for about ten seconds. That is the whole answer. What follows is the shelf-name translation for each country, how to do the food-processor version without accidentally making powder, and the part most guides skip: when crystal size genuinely changes a biscuit, and when it makes no difference anyone could taste.
We ship cookie cutters from the UK to kitchens all over the world, so a steady share of the recipe questions in our inbox begin with "what on earth is caster sugar" — usually from someone in Texas or Ontario halfway through one of our biscuit recipes. Fair question. Here is everything we know.
What caster sugar actually is
Chemically there is nothing special about it. Caster sugar, granulated sugar and icing sugar are all sucrose, identical in sweetness gram for gram; the only difference is how finely the crystals are milled, and that changes how fast the sugar dissolves and how it behaves when creamed into butter. The name comes from the caster — the shaker it was once fine enough to pour through.
| Sugar | Typical crystal size | What that means in practice |
|---|---|---|
| Granulated | 0.4–0.6mm | Slowest to dissolve; leaves visible crystals in low-moisture doughs |
| Caster / superfine | 0.2–0.35mm | Dissolves fast, creams well; the default for biscuits and meringues |
| Icing / powdered | Under 0.1mm, plus 2–3% cornflour | Dissolves instantly but cannot aerate butter; for icings and dusting |
Caster sits in the useful middle: fine enough to dissolve quickly in a dry dough or a meringue, still crystalline enough to cut air into butter during creaming. Icing sugar can do neither job — more on that below.
What it's called around the world
The sugar itself is sold almost everywhere; only the name changes at the border.
| Where you are | What to look for on the shelf |
|---|---|
| UK & Ireland | Caster sugar (golden caster is the unrefined version) |
| United States | Superfine sugar; sometimes baker's sugar or bar sugar |
| Canada | Superfine, berry sugar or instant dissolving sugar, depending on the brand |
| Australia & New Zealand | Caster sugar, same name as the UK |
| France | Sucre en poudre, also sold as sucre semoule |
| Anywhere else | Anything labelled superfine or extra fine; failing that, the food processor |
One warning on volume before you swap anything. Finer crystals pack tighter, so a cup of caster sugar weighs about 225g while a cup of granulated weighs about 200g. Substitute cup for cup and you are roughly 10% short of sugar; substitute gram for gram and the swap is exact. This is one of the better arguments for baking by weight, and our grams-to-cups conversion chart covers the rest of the pantry.
The ten-second caster sugar substitute
No superfine on the shelf? Make it. A food processor or a jug blender both work.
- Weigh out the granulated sugar your recipe needs, in grams, before it goes anywhere near a blade.
- Pulse for about ten seconds total — three or four short pulses rather than one long run.
- Leave the lid on for another thirty seconds. Sugar dust hangs in the air, and opening the bowl immediately lays a fine sticky film over everything within arm's reach.
- Check the texture. It should feel like fine sand between your fingers, with distinct sparkly crystals still visible. That is caster.
Weighing first matters because blitzing shrinks the volume: 200g of granulated measures about a cup going in and noticeably less than a cup coming out, so measuring afterwards under-delivers. And resist the urge to keep the motor running. Thirty seconds or more carries you past superfine into homemade icing sugar, which is a different ingredient with different jobs.
When crystal size actually matters
Meringues and anything whisked
This is the one place we would never use straight granulated. Egg white holds very little water, and large crystals that fail to dissolve reappear later as grittiness, a speckled crust and the syrupy beads that weep from the surface during a slow bake at 100°C (210°F). The test is simple: rub a little of the finished meringue between two fingers. Smooth means the sugar has dissolved; gritty means keep whisking — or start with caster next time and save yourself five minutes.
Low-moisture creamed doughs
Shortbread and cut-out biscuit dough have almost no free liquid — the only water on offer is the 15–18% hiding inside the butter. Granulated crystals never fully dissolve there, and you can see it in the result: a speckled, faintly crackled surface, plus a touch more spread as the surviving crystals melt late in the bake. If your carefully cut shapes keep losing their edges, sugar grind is one suspect on a longer list we have worked through in why do my cookies spread. Our standard cut-out dough — 250g (2 cups) plain flour, 125g (½ cup plus 1 tablespoon) butter, 100g (½ cup) caster sugar and one egg, baked at 180°C (350°F) — specifies caster for exactly this reason; the full logic lives in the biscuit dough that actually holds its shape.
Creamed sponges
When butter and sugar are creamed together, the crystal edges cut millions of tiny air pockets into the fat, and those pockets are what the raising agent later inflates. Finer crystals cut more and smaller pockets, which reads as a finer, more even crumb. A granulated sponge still rises; it just rises a little coarser. Nobody sends one back.
When it does not matter
- Anything with plenty of liquid. Cake batters with eggs and milk, brownies, muffins, pancakes: there is more than enough water to dissolve either sugar long before the crumb sets. Swap freely, by weight.
- Melted-butter methods. Where sugar meets warm melted butter or golden syrup — flapjacks, gingerbread made by the melting method — it dissolves on contact and grind is irrelevant.
- Dough that rests overnight. A long chill gives sugar hours to pull moisture from the dough and dissolve on its own; a granulated-sugar biscuit dough made the night before bakes remarkably close to a caster one.
- Sparkly finishes. Where you want visible crystals — the crunchy coat on a snickerdoodle, the glitter on top of shortbread — granulated or demerara is the better tool. Caster would simply vanish.
What not to swap in
- Icing sugar (powdered or confectioners' sugar) is not a caster sugar substitute. It is milled to a powder and cut with 2–3% cornflour to stop it caking. The starch and the missing crystal structure mean it cannot aerate butter properly, and biscuits made with it turn out dense and pasty. Use it only where a recipe asks for it.
- Demerara and turbinado run the other way: crystals two or three times larger than granulated, plus a coat of molasses. Lovely on top of things, wrong inside a delicate dough.
- Golden caster sugar is the honourable exception: same grind, lightly unrefined, with a faint caramel note. Swap it one for one anywhere — just expect ivory meringues rather than white ones.
The short version: match the crystal, not the label. Weigh out 100g of superfine, berry sugar or ten-second-blitzed granulated and any biscuit recipe on this blog will behave exactly as written, whichever country your oven is in. Sugar sorted and only the shapes missing? The shop is this way.