Was your circle skirt too big when you were finished?
Did you cheat?
Tell the truth, now!
I bet you tried to skip all the math and do this:

Say you wanted the waist hole to be 32 inches.  Cutting the circle can be a real bitch. So you fold the fabric in half twice, and then you divide that measurement by 4.  This gives you 8 inches.  You measure a line that's 8 inches long, because lines are easier.  But then you make a curve, because you really need a circle. 

If you look at the gap between your 8 inch line and the curved line, there are several inches of extra space in there.  Now multiply that space time 4, and that's where you got a skirt that's too big. 

If you really want to skip most of the math, get yourself a bendable ruler. That way, you can make the ruler curved and use that instead of all those calculations.