Confectioner's sugar, also known as icing sugar and powdered sugar, is not truly a different kind of sugar than the regular white sugar you probably have in your kitchen. Instead, it is granulated sugar that has been finely crushed or ground. It also generally has some added cornstarch; it is so fine that it would clump together without this addition.
If the recipe you are making does not require the sugar to dissolve quickly, use regular granulated sugar as a replacement. Because of the size of its particles, granulated sugar will not work well in cases where the sugar needs to quickly dissolve without heat, such as in some icings and sauces. Because confectioner's sugar is finer and lighter than regular granulated sugar, you do not need to use as much regular sugar as confectioner's sugar. Replace each 1.75 cups of confectioner's sugar with 1 cup of regular granulated sugar.
Fruit, Baker's or Superfine Sugar
Fruit sugar, baker's sugar and superfine (or caster) sugar can be substitutes for confectioner's sugar. All three of these types of sugar have smaller particles than regular sugar. Of the three, fruit sugar has the largest particles, baker's sugar is smaller, and superfine or caster sugar is the smallest. The difference between these three types of sugar and confectioner's sugar is that these sugars are granulated, meaning the actual crystals are smaller than regular sugar. Confectioner's sugar, on the other hand, is granulated sugar that has been broken into smaller fragments.
Making Your Own
You can make your own confectioner's sugar in your own kitchen. Mix 1 cup of regular granulated sugar with 1 tbsp. of cornstarch, then grind the mixture into a fine powder in a blender or coffee grinder. When the sugar mixture becomes fine and powdery, your confectioner's sugar is done. You may now use it as you would use store-bought confectioner's sugar in any recipe.