You measure under your bosoms and then over your breasts with a supportive bra on. You subtract the numbers and that is your bust size. For instance, you were 37.5 under so you round that up to 38. Then you measured over your breasts so the difference from 38 to 43 is 5 so you are a 38 DD to 38 E. The difference of your rib cage to the projection of your breasts is your cup size. So, it goes like this.....
1 inch= A
2 inches= B
3 inches= C
4 inches= D
5 inches= E/DD
6 inches= F/G
and etc.
Each bra fits differently so a E in one size would be a DD in another. I am a 34 F, but that is due to the style I wear, I also fit into a 34 DD in Victorias Secret Angels. Alot depends upon the shape of your breasts, so don't try to order online until you find one that fits and you have the style number. Also, for those measuring uneven number under their breasts, or rib cage(this is the band size) round it up to the next even number, band sizes are 32,34,36,38,40 and so on.
Hope this helps. The best way is to go to Victorias Secret or Macys, or even a Brazier for a fitting.