Bra sizes are typically expressed as a band size followed by a cup size. The band size is the measurement around your rib cage, and the cup size is determined by the difference between your bust and your band size.
* Band sizes usually range from 28 to 44, though some brands offer wider ranges.
* Cup sizes are typically A, B, C, D, DD (or D+), E, F, G, H, and so on.
Therefore, a "30B" bra would be an unusual size. You may be thinking of a different combination of band and cup size.