breast–chest difference breast–chest difference breast–chest difference, also known as bust–band difference, is a measurement used for quantifying breast size. it is calculated as bust circumference minus the band or underbust circumference. breast–chest difference is used in the determination of cup size, with 12–14 cm equating to an a cup, 14–16 cm a b cup, 16–18 cm a c cup, and 18–20 cm a d cup. breast–chest difference has been used in the measurement of breast development in transgender women on feminizing hormone therapy.