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With the cost of ingredients going up at the moment, you may be looking at the sugar packets at the supermarket and wondering what the difference is between granulated and caster sugar. Caster sugar is on average 50-100% more expensive than granulated sugar, so is granulated sugar the same as caster sugar?
The simple answer is yes, it's the same sugar, but it's less finely ground.
But let's look at the more complicated answer, or this post is going to be very short!

In the UK we have many different sugars. Here is a list of what you usually find at the supermarket:
1: Granulated - coarse ground white sugar in a bag with blue print.
2: Caster - finer ground white sugar in a bag with yellow or orange print.
3: Icing Sugar - white sugar ground to a fine powder in a bag with pink print.
4: Light Brown Sugar - white sugar with molasses (treacle) mixed through to give a soft, caramel flavour. Usually in a plastic bag with beige print.
5: Dark Brown Sugar - the same as light brown sugar but with more molasses. Usually in a plastic bag with dark brown print.
6: Demerara Sugar - Partially refined sugar which is coarser and harder in texture than light brown sugar, but has a similar colour. Often used to put in coffee. Usually in a bag with red (or sometimes purple) print.
This is in no way a definitive list, as there are also sugar alternatives, muscavados, turbinados, golden etc. All these sugars have different textures, grain size, flavours and ultimately different uses. However, what we're going to talk about today is basic, white sugar. And can you swap one for another.

Can you use Granulated Sugar instead of Caster Sugar?
Absolutely! Granulated Sugar and Caster Sugar are the same white sugar. Caster Sugar has been ground more finely, making the manufacturing process longer, and ultimately the cost higher.
In the UK we are very used to recipe books and chefs advising us to use Caster Sugar. This is because it will dissolve quicker, due to the finer crystal, and give light fluffy cakes and bakes. So, for example when you 'cream' your butter and sugar you are looking for the sugar crystals to start dissolving into the butter, to make it well combined and light. The smaller the sugar crystal, the quicker that takes.
Granulated sugar can easily be swapped out for caster sugar. All you have to do, to make sure your bake isn't compromised, is cream the butter and sugar together for a little longer, giving the coarser grain more time to dissolve.

Will using granulated sugar change my baking?
I'm not going to lie. Swapping out the type of sugar you use may indeed change the texture of your bakes if you don't take the time to learn how to adapt.
The coarser grains of granulated sugar may, if not mix through for longer, create a coarser, less fluffy bake. However, in some bakes this can be a bonus!
For example, using granulated sugar in biscuits and cookies may give them more of a crunch, because the grains will caramelise in the oven. Using granulated sugar in brownies will give them a gooier centre as it won't dissolve as quickly and will remain chewier.
It's really all about knowing how the sugar reacts in your bakes, and why it's there in the first place. Then you can adapt and experiment to get the best results!
Can I use Icing Sugar instead of Granulated or Caster Sugar?
Can Icing Sugar can be used instead of Granulated or Caster Sugar - absolutely not! The grain is so fine it dissolves very quickly and will totally change the texture of your bakes! Icing Sugar has a very specific use, and when it's included in a recipe it is there for a reason.
For more information about the key ingredients in baking read my post Cakes Essential Ingredients.
In the USA you may find that Granulated Sugar is simply called White Sugar, Caster Sugar is called Superfine Sugar, Bakers Sugar or Bar Sugar, and Icing Sugar is called Powdered or Confectioners sugar.