Remove your belt. Measure your waist where you normally wear your pants. (Subtract 1" if you're measuring over clothes.)
Stand with your heels together, and measure around the fullest part of your hips, keeping the tape parallel to the floor.
NO NEED TO SIZE UP!
It's always a temptation to order a larger size, to allow for shrinkage and to provide room for growth. But weve already figured that in. After normal shrinkage from a washer and dryer set on “low”, our clothes will still fit properly, with room for growth spurts.