For instances when your regular size is sold out during a sale, or maybe your normal size doesn’t work in one brand! Sister sizes are different band and cup combinations that are meant to hold the same amount of volume as your current size.
Sister sizing is a method of finding other band and cup combinations that support you and hold your breast volume in the same way that your baseline size does. This is based around the fact that your cup size is actually a ratio - a ratio of how much volume is in your breasts based on your bust and band measurements.
Cups sizes are measured the same on every band size - across the board, a D will always be four inches worth of difference between the band and the bust. Those four inches may look smaller on smaller bands, but bigger on bigger bands. Cup sizes are proportional to your body - therefore, a 32D and a 42D’s breasts will look different.
To find your sister size, we want to look for bra sizes that will still hold your volume, without compromising support and a a good fit. Even if that means trying a different cup size or band size that you’re used to.
We know that the bigger the difference between your band and bust measurements, the bigger your cup size will be. This graphic shows this clearly:
This graphic shows what was explained above - cup sizes grow proportionally to their band. As the band grows, the cup increases in volume, though the measurement (and the cup size) stays the same. These 5 band sizes share the same cup, but their breasts are not the same size.
So, we’ve figured out why sister sizing involves figuring out different band and cup combinations that will still support your volume the way your baseline size does. Doing this is actually a lot simpler than you might expect! We need to find bra sizes that use the same ratio as your baseline size.
To find your sister size, you need to go up in one measurement, and down in the other. Ie, you go down a band size, but go up a cup size. Or, up in the band size, and down in the cup size.
Always: Up one, down one.