Measure your back, from one side to the other side (where the side seam of a tight fitting garment would be) at the level right below your breast. The pad size of the splint is the most important measurement. You do not want the pad of the splint to be too big. If it rests on the front of your belly it does not allow much room to reposition your muscles together. You also do not want the pad to be too small or you will have trouble attaching the elastic arms of the splint. The pad should be resting on your sides in the location where the side seam of a garment would be.