Bra size is a measure of the breast tissue i.e Cup size and the ribcage i.e band size.
The cup size is denoted by alphabets e.g. B, C, DD, E, F, FF, G, GG, H, HH, J, JJ etc
the band size is denoted by numbers e.g. 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, etc.
CUP size + BAND size = BRA SIZE e.g 34+F= 34F