No, generally you cannot use icing sugar as a direct substitute because it contains anti-caking starch that ruins texture in baked goods.
You have your mixing bowl ready, the oven is preheating, and the butter is softening on the counter. Then you realize the jar of caster sugar is empty. You spot a bag of icing sugar in the pantry and wonder if it will save your bake. This is a common kitchen crisis, but swapping these ingredients is rarely a safe bet.
Baking is chemistry, not just cooking. The size of the sugar crystal plays a massive role in how your cake rises, how your cookies spread, and how your meringue stabilizes. While both ingredients bring sweetness, they behave differently under heat and when mixed with fats.
Knowing when you can cheat the rules and when you must stick to the script saves you from wasted ingredients. We will break down exactly why these sugars differ, what happens if you force the swap, and the one simple trick to make your own caster sugar in seconds.
Understanding The Differences Between Sugars
Before you attempt a substitution, you need to understand what is actually in the bag. To the naked eye, they both look white and sweet, but under a microscope, they are distinct ingredients with different jobs.
What Is Caster Sugar?
Caster sugar, often called superfine sugar in the United States, sits right between granulated sugar and icing sugar in terms of texture. It is simply white sugar that has been ground down into very fine crystals. These crystals are small enough to dissolve quickly in batters and meringues but large enough to have rough edges.
Those rough edges are vital. When you cream butter and caster sugar together, the sharp crystals slice into the butter, creating millions of tiny air pockets. These pockets expand in the oven, giving cakes their lift and light texture. Without that physical friction, your sponge cake becomes a dense brick.
What Is Icing Sugar?
Icing sugar, known as confectioners’ sugar or powdered sugar, is pulverized until it becomes a fine dust. It dissolves instantly in liquids, which makes it perfect for glazes and frostings where you do not want a gritty texture.
However, icing sugar is not 100% sugar. Manufacturers add an anti-caking agent, usually cornstarch (cornflour) or tricalcium phosphate, to keep it from clumping into a rock-hard mass in the bag. This additive usually makes up about 3% to 5% of the total weight. That might sound small, but in chemistry terms, it is significant. Starch absorbs liquid and gelatinizes when heated, which alters the structure of whatever you are baking.
Can I Use Icing Sugar Instead Of Caster Sugar? – The Verdict
The short answer remains no for most standard baking recipes. If you swap them one-for-one, you introduce unwanted starch and lose the aeration power of the crystals. The result is almost always a texture failure.
Many home bakers ask, “can I use icing sugar instead of caster sugar?” expecting a simple sweetness exchange. Sweetness is not the issue; structure is. If you use icing sugar in a recipe that calls for creaming butter and sugar, the mixture will turn into a smooth paste rather than a fluffy, aerated base. The cornstarch will also soak up the liquids in your batter (eggs, milk), leaving you with a dry, crumbly final product.
Consider the weight difference as well. Because icing sugar is a fine powder, it packs differently than crystalline sugar. If you measure by volume (cups), you will end up with way more sugar than the recipe intends, leading to an overly sweet, sticky disaster. Even if you weigh it, the lack of air bubbles means your leavening agents (baking powder/soda) have to work double-time, and they usually fail to compensate.
The Chemistry Of The Swap: Why Texture Suffers
When you look closely at baking science, the problems with this specific substitution become obvious. It comes down to three main factors: aeration, absorption, and purity.
The Aeration Problem
Standard recipes rely on mechanical leavening. This happens during the creaming stage. Caster sugar crystals are sharp. They cut through fat molecules, trapping air. Icing sugar particles are smooth and dusty. They dissolve into the water content of the butter immediately. No friction means no air bubbles. No air bubbles mean no rise.
The Cornstarch Interference
That small percentage of cornstarch in icing sugar is a drying agent. In a frosting, this is great because it helps the mixture set and crust over. In a cookie or cake, it creates a chalky mouthfeel. According to baking authorities like King Arthur Baking, added starches can significantly tighten doughs, making tender baked goods tough.
Dissolving Rates
Caster sugar dissolves fast, but not instantly. This delayed dissolving allows the structure of a cookie to set before the sugar fully melts and spreads. Icing sugar melts immediately. In cookies, this causes them to spread too thin, too fast, often burning at the edges before the center is cooked.
When The Swap Might Actually Work
Baking has exceptions. While you should avoid this swap for sponges and meringues, there are specific scenarios where using icing sugar instead of caster sugar is acceptable or even preferred.
Shortbread And Melting Moments
Shortbread cookies rely on a “short” texture—crumbly, tender, and melt-in-the-mouth. Many recipes actually call for icing sugar specifically because you do not want significant gluten development or air pockets. If you are making shortbread and only have icing sugar, you are likely safe, though the texture will be slightly more powdery than usual.
No-Bake Cheesecakes
In desserts where you do not apply heat, the dissolving factor is the priority. If you are making a no-bake cheesecake filling, caster sugar can sometimes leave a grainy texture if it doesn’t dissolve fully in the cream cheese. Icing sugar works perfectly here, and the cornstarch can actually help stabilize the filling slightly.
Simple Glazes
This is the native home of icing sugar. You can never use caster sugar for a glaze unless you dissolve it in hot liquid first (making a syrup). For a simple mix-and-pour glaze, icing sugar is the only option.
Common Baking Disasters Caused By Substitution
To illustrate why you should avoid this, let’s look at what happens to three classic baked goods if you make the mistake of using icing sugar instead of caster sugar.
1. The Meringue Meltdown
Meringue relies on the bond between egg protein and sugar. Caster sugar dissolves into the egg whites while creating a stable lattice. Icing sugar dissolves too fast and the cornstarch makes the meringue “weep” (leak liquid) later. Your pavlova will be soft, chalky, and likely collapse in the oven.
2. The Sponge Cake Brick
As mentioned, without the creaming process, a Victoria Sponge will not rise. You will end up with a dense, rubbery disc that feels heavy in the stomach. The crust will also be different; caster sugar aids in caramelization, giving that golden brown exterior. Icing sugar tends to burn faster or create a pale, soft crust.
3. The Spreading Cookie
If you use icing sugar in chocolate chip cookies, they will likely spread across the baking sheet into one giant, thin wafer. They will lack the chewy center and crispy edge contrast that granulated or caster sugar provides.
The Best Solution: Make Your Own Caster Sugar
If you have regular white granulated sugar in your house, you do not need to panic. You can manufacture caster sugar in less than a minute. This is the only safe substitution for baking.
Granulated sugar is chemically identical to caster sugar; the crystals are just larger. You simply need to reduce their size. This method works perfectly because you are not adding any starch to the mix.
Step-By-Step Instructions
- Measure your sugar — Weigh out the amount of granulated sugar equal to the caster sugar required in your recipe.
- Use a clean processor — Ensure your food processor or blender is completely dry. One drop of moisture will turn the sugar into syrup.
- Pulse gently — Pulse the sugar in short bursts. Do not let it run continuously, or the heat from the blades might melt the sugar crystals.
- Check the texture — Stop when the sugar looks like fine sand. If you go too far, you will accidentally make icing sugar (powder).
- Let dust settle — Wait a few seconds before opening the lid to avoid inhaling a cloud of sugar dust.
You can use this DIY caster sugar immediately in any recipe with zero adjustments to weight or volume.
Adjusting Measurements If You Must Swap
If you are in a situation where you absolutely must use icing sugar instead of caster sugar (perhaps for a dense brownie where aeration matters less), you cannot just use the same cup measure. The density difference is massive.
Weight Is King
Always use a digital scale. 100g of caster sugar provides the same sweetness as 100g of icing sugar. However, 1 cup of caster sugar weighs approximately 200g, whereas 1 cup of icing sugar weighs only about 120g. If you swap by cups, you will be adding nearly half the amount of sugar needed, resulting in a bland, bread-like texture.
If you substitute by weight, you solve the sweetness issue, but you still have the starch problem. Expect the result to be drier. You might need to add a teaspoon of milk or water to the batter to offset the absorption from the cornstarch.
Sugar Alternatives Comparison Table
Here is a quick reference to help you decide which sugar to grab from the cupboard.
| Sugar Type | Dissolve Rate | Best Uses | Contains Starch? |
|---|---|---|---|
| Caster Sugar | Fast | Sponges, Meringues, Mousses | No |
| Icing Sugar | Instant | Frosting, Glaze, Shortbread | Yes (usually) |
| Granulated Sugar | Slow | Cookies, Hot Drinks, Syrups | No |
| Brown Sugar | Medium | Chewy Cookies, Rich Cakes | No (contains molasses) |
Using the wrong column here essentially guarantees a different chemical reaction in your oven. It is rarely worth the risk for standard baking.
Metric Vs. Imperial Confusion
Recipes from different parts of the world complicate this issue. British recipes almost exclusively call for caster sugar for baking. American recipes often call for granulated sugar for the same tasks. This is because American granulated sugar is slightly finer than British granulated sugar, so it creams reasonably well.
If you are in the US and a UK recipe calls for caster sugar, you can usually get away with standard granulated sugar, though the texture might be slightly coarser. However, you still cannot use powdered sugar (icing sugar) as the substitute. The rule remains consistent regardless of the region: crystal sugars swap with crystal sugars; powder sugars stay with glazes.
Correcting A Mistake Mid-Bake
If you accidentally used icing sugar instead of caster sugar and the batter is already mixed, you have limited options. You cannot remove the sugar. The best approach is to manage your expectations.
Do not throw the batter away. Bake it, but keep an eye on the time. The batter may bake faster due to the lower moisture content. The resulting cake will likely be denser, similar to a pound cake or a muffin texture. You can salvage it by soaking the warm cake with a simple syrup (sugar and water boiled together) to add moisture back in, or serve it with custard or ice cream to mask the dry texture.
For cookies, if you used icing sugar, expect them to be more tender and cake-like. They won’t be bad, just different from the classic chewy cookie you intended.
Key Takeaways: Can I Use Icing Sugar Instead Of Caster Sugar?
➤ Icing sugar contains anti-caking starch that dries out baked goods.
➤ Caster sugar crystals are essential for trapping air during creaming.
➤ Substituting by volume (cups) leads to drastically lower sugar levels.
➤ Simple glazes and shortbread are the only safe places for this swap.
➤ You can make caster sugar by pulsing granulated sugar in a blender.
Frequently Asked Questions
Is powdered sugar the same as icing sugar?
Yes, they are the same thing. In the US, it is called powdered or confectioners’ sugar. In the UK, Australia, and New Zealand, it is called icing sugar. Both contain finely crushed sugar mixed with a small amount of cornstarch or potato starch to prevent clumping.
Can I use granulated sugar instead of caster sugar?
Yes, this is a much safer swap. Granulated sugar crystals are larger, so they may take longer to dissolve and might result in a slightly speckled crust on cakes. However, the chemistry is identical, and your cake will still rise properly, unlike with icing sugar.
Does icing sugar make cookies spread more?
Usually, yes. Because icing sugar melts instantly in the oven, the dough structure liquefies before the proteins and starches set. However, some specific “melting moment” recipes use this trait intentionally to create a very tender, delicate cookie that dissolves in the mouth.
What is golden caster sugar?
Golden caster sugar is unrefined caster sugar. It retains some molasses, giving it a pale golden color and a subtle caramel flavor. You can swap golden caster sugar for white caster sugar one-for-one without any changes to the recipe’s texture or chemistry.
How do I store homemade caster sugar?
Store your DIY caster sugar in an airtight container immediately. Because it lacks the anti-caking agents found in icing sugar and has more surface area than granulated sugar, it absorbs moisture from the air quickly and can turn into a solid brick if left exposed.
Wrapping It Up – Can I Use Icing Sugar Instead Of Caster Sugar?
Baking requires precision, and ingredients usually serve a structural purpose beyond just flavor. While it is tempting to use whatever white powder is in the cupboard, substituting icing sugar for caster sugar is a recipe for dense, dry, and disappointing results. The lack of friction for aeration and the addition of drying starches fundamentally changes how your batter reacts to heat.
Save the icing sugar for the decoration on top. If you need caster sugar in a pinch, reach for the granulated sugar and your blender instead. It is the only way to ensure your sponges stay light, your meringues stay stiff, and your kitchen stays disaster-free. For more insights on food science and ingredient interactions, resources like the USDA Food and Nutrition pages offer great data on component breakdown.