Rash guards for men tend to come in sizes small through extra-extra-large. The exact sizes of the shirt will depend on the specific manufacturer, but typically you will need to know your height, weight and waste size to determine the proper fit. Measure your waist by placing a tape measure around your stomach against the skin, just below your navel. Refer to the size chart of your particular manufacturer to determine proper size. Using the size chart on the O’Neill Water Outfitters website as an example, a man weighing 145 pounds, who is 5 feet 9 inches tall and has a waist measuring 30 inches, would require a size small rash guard. Likewise, if you weighed 200 pounds, were 6 feet tall and had a waist of 36 inches, your size would be a large.
Women should factor in their bust measurement, in addition to weight, height and waist, when determining the proper size of a rash guard. To measure the bust, wrap a measuring tape around your bra strap line; the tape should go under your arms and be against the skin. According to the O’Neill size chart, for example, a woman weighing 120 pounds, who is 5 feet 6 inches tall, has a waist of 26 inches and a bust of 33 inches, would wear a size small top.