I cannot for the life of me figure that out. I have x-rays of my pelvis which indicate its only 4" but that can't be right. Mimi sent me instructions for measuring but I just can't tell where to measure. But what I do know is my sit bones are well supported on the terry butterfly which is 6.1" wide, so I am thinking either of these saddle should be wide enough. I do plan to call Wallingford tomorrow and talk to them abou it. Does Diane also know saddles as well as Bill, might be easier to explain my problems to a girl.....