Can you use granulated sugar instead of powdered sugar in cake frosting?

If you put the granulated sugar into your blender & turn it into powdered sugar you can use it. Just blend the amount the recipe needs. Like, if it says 1 cup powdered, use one cup granulated. The problem with using regular sugar is - the frosting will sweat. It will look ok at first but when it sits for a while the frosting will look like it has water puddles on it. If you plan to eat it soon after its made, that would be ok. But it shouldn't sit for very long. You might want to find a recipe that uses regular sugar.