Most women come in wearing the wrong size bra.The  sizes are too big in the band and too small a cup size. So, a good starting point is to go down a band size and up 2 cup sizes and see how that size feels. For example, if you are wearing a 40DD go to a 36-38G ), You go up 2 cup sizes because when you go down a band size, the same cup size will actually be smaller as well.