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Bra Sizes Explained

Bra size has two measurements; the band (ribe cage) and the cup (bust).

Cup size is not the same as breast size but instead represents how much bigger the breasts are than the rib cage (the band). It is therefore not possible to know how big someones breasts are by their cup size alone, because it depends on the band size. For example, a 28D is smaller in size than a 40D.

How To Measure Bra Size

Band Size


The band size is measured numerically e.g. 30, 32, and 34…etc. and refers to the size of the rib cage.

Cup Size


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.

Bra Sister Sizes

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 grey cells demonstrate how the sizes that diagonally adjacent from the top right to the bottom left are the same in breast volume 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 same 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.

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