Caster Sugar — Cups to Grams

What Makes Caster Sugar Different — and Why the Weight Per Cup Matters

Caster sugar is granulated white sugar milled to a crystal size of approximately 0.35mm — roughly half the crystal diameter of standard granulated sugar (0.5–0.6mm). This is not a chemical or compositional difference: both are pure sucrose. The difference is entirely in particle size, and that particle size has measurable consequences in both measurement and baking behavior.

Because the finer crystals pack together with less interstitial air space, a level cup of caster sugar contains approximately 225 grams versus 200 grams for granulated white sugar. This 25-gram (12.5%) difference is not trivial in a recipe where sugar quantities are calibrated to specific ratios. A Victoria sponge that calls for 225g caster sugar and receives only 200g (because you measured a cup of granulated) will have a marginally less sweet, less moist, potentially less even crumb — though the difference may be subtle depending on the recipe's sensitivity.

The practical implication: when a British or Australian recipe specifies "caster sugar" and gives a cup measurement, convert to grams using 225g per cup. When it gives a gram measurement, use exactly that weight regardless of what type of fine-granulated sugar you have. The chemical identity is the same; only the volume differs.

How to Make Caster Sugar at Home

Commercial caster sugar is milled to a precise particle size in industrial grinders. The home approximation using a blender or food processor works well for most applications, with a few important technique points.

Method: Place granulated white sugar in the bowl of a food processor fitted with the metal blade, or in a high-powered blender. Do not fill more than halfway. Pulse 8–12 times in 2–3 second bursts. After each pulse, open and check: you want the crystals visibly finer and more uniform, but still granular — not yet powdery. Total processing time is typically 20–30 seconds depending on your appliance's power.

The test: Rub a pinch between your thumb and forefinger. Granulated sugar feels distinctly gritty, like coarse salt. Properly processed caster sugar feels like fine beach sand — smooth but still with a slight grain. If it feels like talcum powder, you have made powdered sugar (which will not work as a substitute in most caster sugar applications).

Yield: 200g (1 cup) of granulated sugar produces approximately 190–195g of home caster sugar after processing (a small amount is lost to static clinging to the bowl and blades). The volume increases slightly because the finer crystals create more aeration during processing, but weight is what matters. Measure the finished caster sugar by weight, not volume, if precision is required.

Limitations: Home-processed caster sugar is slightly coarser and less uniform than commercial caster sugar. For most recipes this is acceptable. For very delicate meringues or macarons where precise dissolution rate matters, commercial caster or superfine sugar gives more consistent results.

Caster Sugar in Specific Applications

Pavlova and meringue is the highest-stakes application for caster sugar. The recipe mechanics: beaten egg whites form a protein foam; adding sugar stabilizes this foam by binding water in the protein matrix and eventually dissolving into a thick syrup that reinforces the foam walls. Granulated sugar dissolves too slowly and can create a grainy texture or "weep" (pools of liquid under the meringue) as undissolved crystals continue drawing moisture. Standard ratios: French meringue at 50g caster sugar per large egg white; Swiss meringue at 60–70g per egg white (higher ratio, more stable).

Victoria sponge and génoise rely on caster sugar for its aeration properties in the creaming stage. When caster sugar is beaten into softened butter at medium speed for 4–5 minutes, the fine crystals cut into the fat, creating microscopic air pockets. The result is a pale, fluffy mixture that has increased in volume 50–75% from the starting weight — this is the structural leavening of a butter cake. Granulated sugar produces fewer, larger air pockets, resulting in a denser crumb.

Crème caramel and caramel sauce benefit from caster sugar because it melts faster and more evenly than granulated sugar. Granulated sugar can develop hot spots during caramelization that burn while other areas remain unmelted. Caster sugar's smaller crystals begin melting more uniformly. For dry caramel: heat caster sugar in a heavy-bottomed pan over medium-high heat, swirling (not stirring) until fully melted and amber (165–175°C). For wet caramel: dissolve in a small amount of water first.

Shortbread traditionally uses caster sugar rather than granulated. The finer crystals distribute more evenly through the butter-heavy dough and produce a more tender, even texture. Use at a typical ratio of 75g caster sugar per 225g plain flour and 170g butter for classic Scottish shortbread.

Caster Sugar vs Granulated: Volume and Weight Comparison

The right column illustrates why cup-for-cup substitution between caster and granulated sugar introduces real recipe errors. A cake using 2 cups of caster sugar that is made with 2 cups of granulated sugar uses 50g less sugar — enough to noticeably affect sweetness, moisture, and browning in a recipe not engineered to handle the variance.

Common Questions About Caster Sugar

• How many grams in 1 cup of caster sugar?

  1 cup of caster sugar weighs 225 grams. This is 25g more per cup than granulated white sugar (200g) because caster sugar's finer crystals pack more densely. The weight difference is purely due to particle size — both are 100% sucrose. Always convert by weight when substituting between the two.

• What is caster sugar in US terms?

  Caster sugar is sold in the US as "superfine sugar" or "baker's special sugar." It is available in specialty cooking shops. If unavailable, pulse granulated sugar in a blender for 20–30 seconds until visibly finer but not yet powdery. Stop before the sugar becomes talcum-powder texture — at that point you have made powdered sugar, which is not a substitute.

• Can I substitute granulated sugar for caster sugar?

  Yes — by weight. Use the exact same gram amount and accept minor texture differences (slightly coarser crumb in cakes, marginally less stable meringue). Never substitute cup-for-cup: 1 cup granulated = 200g, but 1 cup caster = 225g. For meringues and pavlova, granulated sugar risks grittiness and weeping — blending it briefly first is strongly recommended.

• Why use caster sugar in meringue?

  Caster sugar's finer crystals dissolve more completely in beaten egg whites, producing a stable, glossy meringue without grittiness or syrup weeping. Granulated sugar dissolves more slowly — undissolved crystals remain in the foam and continue drawing atmospheric moisture after baking, causing weeping. Standard ratio: 50g caster sugar per large egg white for French meringue.

• Is caster sugar the same as icing sugar?

  No. Caster sugar is finely granulated (0.35mm crystals) and weighs 225g per cup. Icing (powdered/confectioners') sugar is ground to a fine powder (<0.1mm) with 3% cornstarch added, and weighs only 120g per cup. They are not interchangeable — powdered sugar contains starch that affects texture in batters, and its extreme fineness creates different dissolution and coating behavior.

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