is depends on the back size aswell as the cup size hun, someone who is a 30DD has alot smaller cup size than someone who is a 38DD, is not all about the cup xx
you best bet is to go into la senza,debhennams etc to have a bra fitting, what size in inches to you measure under your bust? and over your bust at the fullest part xx
your best bet is to go into a shop and get measured hun, coz you may find that each shop you may measure different depending on the style and fit of the bra, i vary from a few cup sizes depending on what style of bra i wear xx