the correct way is, to measure your ribs, directly under your boobs and add 4 inches if even number and 5 if odd ie rib cage says 30, so choose 34 bra, 31 will be 36, then measure around the fullest part of your boobs and for each extra inch will give you cup size ie if you have chosen 34 bra and your mounds measure 34 then you will be a D, odd numbers will give you DD. You will need to try a couple of sizes, say 34dd and 36d as all shops come up slightly different, same as buying tops/ trousers etc. hope that is helpful. x