Bra sizes that are the same in volume are often referred to as “sister sizes”. The grey cells in the table below demonstrate how the sizes diagonally adjacent from the top right to the bottom left are the same in breast volume i.e. the same volume of breast tissue spread across different size rib cages (band sizes).

Sister sizes demonstrate that cup size doesn’t indicate breast size but instead the difference between the breasts and rib cage. If someones rib cage gets larger but their breasts stay the same, they’ll need a larger band and a smaller cup, demonstrating that the difference between their breasts and rib cage has decreased. For example, if someone usually wears a 26H bra and gains weight around just their middle and not their breasts they may need a 28GG instead; the same volume as a 26H but on a larger rib cage.

Bra sizes can be confusing and so if you try on a bra that fits your breasts very well but doesn’t fit in the band then simply remember that: