Gramma is right, of course. I'd add that you probably have a pattern with some different stitch patterns in it, or ribbing, and they'd measure differently. The stockinette part is the easiest to measure and be sure of your gauge. The rest should be fine so long as your tension is about the same.
A slouchy hat is a good choice for a new knitter. If the ribbing (or other bottom edge) fits right, the rest of it can be more-or-less and still look good and be wearable!