A 36-inch waist is closest to a size 20 on the standard size chart and a size 16W on the women's plus-size chart, according to WomensSizeChart.com. In men's sizes, a 36-inch waist is a size large.
Women's sizes are determined not only by waist measurement, but also by hip and bust measurements. Different manufacturers cut their clothes in different ways, so a size 20 from one designer might be a completely different size for another designer. Some clothing stores use their own vanity sizing. For example, a 36-inch waist at Chico's is a size 3. When shopping online, always check the size chart for the specific store or designer to ensure ordering the correct size.