Underwear size measurements typically refer to waist measurements. Most brands measure waist size in inches and divide it into small (S), medium (M), large (L), and extra-large (XL) sizes. You may also see numbers used to designate the size. It is important to understand that different brands have different measuring systems and individuals have different body shapes, so your size will vary from brand to brand. To make sure you get the right fit, measurement is key. Depending on the style of underwear, measure either your waist or hip. Make sure the measuring tape is snug and sit close to the body, not loose. If a particular style isn't working for you, try a different size or brand, or visit a store if you need help with sizing.