Check the size information on the lable or package. It will give a size number based on the hip measurements. That's a bit differennt from your waist measurement, but it's a place to start. The best advice has been already given: try them on. Depending on your style preferance, you will need some trial and error. I have a 34" waist, and I find that size 6 full-cut briefs fit me best, however size 7 hi-cuts generally fit better that 6.