, at least those used in the US
, is a combination of cup size
and rib cage
circumference. The number in the size is arrived at by the distance around the rib cage, plus 5 until a circumference of 33 (add 3 if larger than 33). The cup size is arrived at by measuring how much bigger the measurement around the fullest part of the breast is than around the rib cage. Cup sizes vary between bra manufacturer
s, but usually an A cup is 1 inch bigger around the breasts than the ribs, B cup is 2 inches, C is 3, D is 4, DD (or E in some systems) is 5.
Because of this, breasts needing the same bra size may look very different on different women due to height and build. I found it rather amusing that three college friends of mine all wore a 36B, one 4 foot 8 inches and considered voluptuous by the guys we knew, one 5 foot 1 and thought average-breasted, and one probably about 5 foot 7ish and described by a male friend as "flat as a fourteen-year-old boy."