I did as instructed in Product Details, measured where I would wear the belt then add 2 inches, fits perfect as pictured. I will be wearing it around the hips as a part of my Ren Fair ensemble and the M fit is just right. For those who usually wear M in pants and want a tighter fit higher towards the waist, I suspect the S would work better, though measure for yourself! I may have to puncture one or two extra holes as I'm already wearing it on the tightest hole, but that's not a large deal. The thin, double belt style  is really cute, exactly what I was looking for.