I am a strange case where the Small is too tight and the Medium is too loose. That being said, opt for the size that follows your regular belt size (one size larger than your pants). I wear a 32-33 waist and thought the Small would work, but squeezes a little too much. Medium is slightly too big at the tightest possible placement for the Velcro. I'd honestly say that for the money I cannot complain too much since it does what it's supposed to in masking the ostomy bag and supports it quite well! Between the two sizes in the support sense, I actually forgot I had an ostomy bag at all.