For Men: Match your pants waist size to the chart shown on the product page. If you wear your pants big or tight, you can follow theses instructions on how to get the exact right fit: Measure low around the waist, above the wide point of the hips, about 1 inch above the top of the pubic bone. Once you have that measurement, subtract 2 inches and compare the result to the sizing chart on the product page.  Example: Measurement 1 inch above the pubic bone = 33 inches -2 inches = 31" 31" = Small on the size chart If you are between sizes on the chart, it is best to go with the smaller size for the MInimalist Comfort-Truss, and go with the higher size in the HD version. The Minimalist version stretches a lot more than the HD version which is why we make this recommendation.  For Women: Since women's bodies are much different than men, to make sure you get the correct size, you should measure low around the waist, above the wide point of the hips, about 1 inch above the top of the pubic bone. Then you can match that measurement to the corresponding waist size on the size chart referenced above and shown on the product pages. If you are between sizes on the chart, it is best to go with the smaller size for the MInimalist Comfort-Truss, and go with the higher size in the HD version. The Minimalist version stretches a lot more than the HD version which is why we make this recommendation.