In the United States, granulated sugar can be used in place of caster sugar in some recipes. This is especially true for recipes that require butter and sugar to be creamed together.
In the United Kingdom, however, granulated sugar can usually not be used as a replacement for caster sugar, as the granules are too large to yield the desired consistency. Caster sugar can be created by grinding granulated sugar in a blender or food processor, since caster sugar is very finely ground sugar. Do not grind the sugar too long, as it can become powdered sugar, which is even more finely grained.