Bra size is composed of two dimensions; the band size and the cup size.
The band size is measured numerically e.g. 30, 32, and 34…etc. and refers to the size of the rib cage.

The cup size is measured alphabetically e.g. A, B, C, D, E, F… etc. and refers to the size of the breasts in relation to the rib cage much bigger the breasts are than the rib cage.

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 are the same breast volume, but with different circumferences i.e. same volume breasts spread across different size rib cages.


Sisters sizes demonstrate that if your bra size provides the correct cup volume but you want to change the band size, you will also need to change your cup size accordingly, and vice versa. For example, if a woman who normally wears a 36D finds that the band is too loose and so tries on a 34D, the cups will be too small. To obtain the same cup volume as her 36D bra she will need to try a 34DD.