When selecting a size for pants, the waist may seem like an obvious place to start, but it can be misleading. Your pants waist may not be at the same point as your natural waist which is usually located somewhere around your belly button. When you choose your size for a dress you compare your measurement with the sizing chart and then finished garment measurements and this usually works out ok. However, as the silhouette of pants have changed to sit lower on your hips your actual waist measurement becomes less valid. For instance, the Manhattan trousers are designed to hit approximately 3-4″ below your bellybutton. As this is closer to the hips it means the waist will be bigger.
My recommendation in choosing a pant size is to select based on your hips. This point of measure matters the most as it is your widest point and you don’t want to skimp. It is much easier to make a pant smaller than bigger. Make your waistband as a muslin and then determine what modifications, if any, need to be made for a secure fit.