There’s nothing like the support you get from a beloved sister to help you feel your most comfortable. And it is the same when it comes to your bra. Whether you’re a 34D or a 38F, your normal bra size comes from a family of sisters, related bra sizes who share one important quality in common - the volume they hold in the cup.
While dress sizes go up in number the bigger, they get, bra sizes don’t work like that. In fact, they’re more like ratios because the volume of the cup size (the amount of breast tissue it can hold) varies with the band size.
Let’s say you’re a 34C and you try on a bra. You find the cup size is fine, but the band’s a bit too tight. If you didn’t know about sister sizes, you would probably opt to try a 36C next. But because you’ve gone up a band size, you’ve also gone up a cup size without even realising it! Instead, you should try a 36B, because while there’s more room in the band, the cup volume is the same. In this way, sister sizes are far more accurate.
To give you another example, if you’re a 38C, your bra belongs to this family: 30F, 32E, 34DD, 36D, 38C (your size), 40 B. So, your closest sister sizes are 36D and 40B, because while the band may vary, the cup volume is the same. You’d also find that 30F, 32E and 34DD would fit you in the cups, too. So, a 38C has the same bra cup size as a 30F. Mind-blowing, right?