I'm currently wearing a 36f. I measure 33.5/34 cms under and 40cms around the fullest part which I think makes me a 34f but when I tried it on it hurt and my boobs were all wrinkled up and out. The m&s lady measured me as a 38f, but I wasn't convinced on her 'fitting' as it felt too loose. Need to buy at m&s as have a load of vouchers to use.