To be honest hun,I just guessed,I wanted to be dd so I bought 34 and 36 backs dd,and just hoped they fit,lol,the 36 were so comfy,and had a little more room in the nipple area because they were too big,but omg bliss on a night,then I wore the 34 back when the swelling went down.xxx When I got measured properly weeks later I was 34d.xxx