I ordered a small, and the bra came with a sticker that had an "A" on it. If you are an "A" cup a small will be perfect! "B" cup a medium etc. I also compared one of my bras to the strapless bra, and you can tell that it is close to an A cup as well! Only thing I have to complain about is the drawstring in the middle. It looks a little cheap but have yet to try it on! At the end there is a tie, that holds the string together to make sure the fit is just right!