Behold the power of Wikipedia, wherein it can even be discovered how bra sizes actually work:

The size of breasts is often expressed in terms of the size of the bra. This is measured as follows: Two measurements are taken, the first is a circumference of the body with the tape being placed under the breasts. This provides the "band size." The second measurement is similar, but includes the breasts. The first result is then subtracted from the second. A difference of 1 inch requires an A cup size; 2 inches, a B cup; 3 inches, a C cup; and 4 inches, a D cup. Therefore, a woman who has a band size of 36 inches, and a measurement over her breasts of 39 inches, would be best served by a bra size of 36C.

Part of me is going "ummm... eh?", but the other part is going "heheheheheheheheh... you said cup... hehehehehehehe."