To find your natural waist, bend to one side and note where your torso creases — that's the narrowest point. Measure there, snug but not pulling tight.
The mismatch between your natural waist and a jeans waist size is a frequent source of confusion: low-rise jeans are sized at the hip, so their "waist" number can be several inches larger than your measured waist. Rise is what reconciles the two.