About 70% of women wear ill-fitting brassieres. Good fits are calculated thus: measure around the chest directly under the breasts and add four inches to the number if it is even and five inches if it is odd. This is the bra size. To determine cup size, subtract the bra size from the bust size (around the fullest part of the bosom). Differences of 0-3 inches equate to, respectively, cup sizes A to D, while 10 inches give you a G-cup.