Bra size is composed of two dimensions; the band size and the cup size:
It is important to note that the cup size doesn’t represent volume, but instead represents how much bigger they are than the rib cage. Breast size cannot be determined by the cup size alone as it is affected by the band size. For example, D cup breasts on a 28 band are smaller in volume than D cup breasts on a 40 band.
Bra sizes that are the same in breast volume are often referred to as “sister sizes”. The table below shows a range of bra sizes. The cells that are diagonally adjacent from the top right to the bottom left (e.g. the grey cells) are the same in breast volume but across different circumferences i.e. the same volume of breast tissue spread across different size rib cages.
Sisters sizes demonstrate that if your bra cups fit your breasts perfectly but you need a different band size (because it is too tight or too loose), you will need to change the cup size as well to achieve the came fit in the cup. For example, if a woman who normally wears a 36D finds that the band is too loose but the cups fit perfectly, she will need to try a 34D to gain the same cup volume.