What Is Caster Sugar? Substitutes, Uses, and Differences

Anyone who’s cracked open a British baking book has seen “caster sugar” listed and wondered whether it’s worth a special trip. The short answer is that caster sugar is simply white sugar ground to a finer crystal — about 0.35 mm compared to 0.5 mm for standard granulated sugar, as King Arthur Baking (trusted baking resource) explains. The difference matters most when you need sugar to disappear quickly into a cold batter or a delicate foam, and this guide will help you decide when to reach for the fine stuff.

Granulated sugar crystal size: 0.5 mm ·
Caster sugar crystal size: 0.35 mm ·
Icing sugar crystal size: 0.04 mm ·
Common US name: Superfine sugar ·
Common UK name: Caster sugar

These differences in crystal size set the stage for every substitution decision that follows.

Is caster sugar just normal sugar?

What is caster sugar made of?

• Both caster sugar and granulated sugar are derived from sugarcane or sugar beets, with no added ingredients (King Arthur Baking).
• The sole difference is the size of the crystals.

How does the grain size differ?

• Caster sugar crystals measure about 0.35 mm; granulated sugar crystals are about 0.5 mm (Bakers Authority).
• This smaller size means caster sugar dissolves 30–40% faster in cold liquids, a critical advantage in recipes like meringue or simple syrup (King Arthur Baking).

The implication: If you’re making a cold mixture or a foam where every second of mixing counts, the finer grind can be the difference between a silky meringue and a gritty one. For hot liquids or long mixing times, the difference fades.

What can I substitute for caster sugar?

Can I use granulated sugar instead?

• Yes, by weight they are interchangeable. The Hummingbird Bakery (London-based bakery) states that cakes made with granulated sugar hold up because the flavor and water content are identical.
• For best results, blitz granulated sugar in a food processor for 30 seconds to mimic caster texture (King Arthur Baking).

Can I replace caster sugar with brown sugar?

• Yes, but note that brown sugar contains molasses, which adds moisture and a caramel flavor (The Hummingbird Bakery).
• In recipes where a brown sugar taste is welcome (e.g., cookies, spice cakes), it works well, but the texture may be slightly denser.

Can I use powdered sugar as a substitute?

• Powdered sugar (also called icing sugar) is ground to a much finer powder (0.04 mm) and contains about 3% cornstarch as an anti-caking agent (Bakers Authority).
• The cornstarch can alter the texture of baked goods, making them denser or preventing proper aeration. It is not a direct substitute unless the recipe is adjusted.

The trade-off: Granulated is the safest swap; brown sugar works when flavor permits; powdered sugar should be avoided in delicate cakes but can work in icings and no-bake desserts where cornstarch is welcome.

Can I turn regular sugar into caster sugar?

How to make caster sugar at home

1. Place 1 cup (200 g) of granulated sugar in a blender or food processor.
2. Pulse for 30–60 seconds until the crystals are visibly finer but not dusty. Bakers Authority recommends short pulses to avoid turning it into powdered sugar.
3. Let the dust settle for 30 seconds before opening the lid — fine sugar particles can create a cloud.
4. Store in an airtight container for up to 6 months.

The catch: Homemade caster sugar is slightly coarser than store-bought, but for most home baking the difference is negligible.

What is caster sugar called in the USA?

Is it also called superfine sugar?

• Yes. In North America, caster sugar is commonly sold as “superfine sugar,” “baker’s special sugar,” or “extra-fine sugar” (King Arthur Baking).

Is caster sugar available in American supermarkets?

• Some large chains stock superfine sugar in the baking aisle, but it is not as ubiquitous as in the UK (My Frugal Home (budget cooking site)).
• Many American bakers simply make their own using the method above.

What this means: If you are in the US and a recipe calls for caster sugar, check the baking aisle for “superfine sugar” first. Not finding it is not a crisis — a blender and 30 seconds solve the problem.

Is caster sugar powdered sugar?

What is the difference in texture?

• Caster sugar feels like fine sand; powdered sugar feels like talc.
• Powdered sugar is ground to 0.04 mm and includes cornstarch; caster sugar is about 0.35 mm with no additives (Bakers Authority).

Can I use powdered sugar instead of caster sugar?

• Not recommended for most cakes because the cornstarch can interfere with creaming and gluten development (The Hummingbird Bakery).
• In frostings, glazes, or dustings, powdered sugar works fine, but the substitution is not 1:1 by volume due to the difference in density.

The pattern: Caster and powdered sugar occupy entirely different physical states. Replacing one with the other changes the chemistry of the recipe more than most home bakers expect.

Two attributes, one pattern: the crystal size governs solubility, which in turn determines which recipes tolerate a swap and which do not. The table below sums up the differences.

The implication: Choosing the wrong sugar for a delicate cake or meringue can push the texture from airy to dense in one swap.

“Caster sugar is much finer and dissolves much more easily than granulated sugar, making it ideal for cakes and custards where you want a smooth texture.”

— The Hummingbird Bakery (London bakery and cookbook publisher)

“Superfine sugar — also called baker’s special sugar, extra-fine sugar, or caster sugar — dissolves very quickly, making it a good choice for meringues, cold liquids, and any recipe where sugar needs to dissolve without heat.”

— King Arthur Baking Company (trusted baking resource since 1790)

For home bakers in the US, the choice is clear: either find superfine sugar in the baking aisle or make your own in under a minute. For UK bakers, caster sugar is a pantry staple worth keeping on hand — but if you run out, granulated sugar blitzed for 30 seconds will save your sponges without a trip to the shops.

Understanding the differences between various kitchen staples, such as caster and kosher salt conversions, can help home bakers achieve consistent results.

Frequently asked questions