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Yeah, so the band size is "how far around are you?" In American sizing, it's in inches. I'm a 36, meaning my chest, including breasts are 36 inches around. And then cup size is calculated by the difference between band and the measurement of the chest just below the breasts. A cup is a difference of 1, B is a difference of 2, C is a difference of 3, and so on
So I'm a 36B, meaning my whole chest is 36 inches around, and there's a difference of 2 inches between my chest counting and not counting the breasts.