Everything to Know About Bra Sister Sizes

For me, shopping for a new bra and underwear is, admittedly, a bit of a rarity. Once I find a bra that is comfortable and functional, I use it for years on end. While the main reason behind my infrequent bra shopping sprees is the fact that I only need a couple versatile bras to get the job done, another factor is finding the perfect bra size and bra sister size. It can be tricky to find the right lingerie size when you have to take band width, cup size, and breast size all into account. Additionally, our bodies fluctuate so often, and different brands have different sizing charts and definitions. All in all, it can just make the task seem more complicated than it's worth. But if you can believe it, bra sister size charts help make the fit process a little bit easier.

Unlike the traditional bra sizes that you may be familiar with, like 28C or 42A, sister sizes take into account volume. Basically, they are sizing groups, equivalent to bra sizes, that are based on cup volume. And the volume of cup size varies with the band width. It's easy to calculate your sister bra size. If your band is too tight, go up a band size but go down a cup size. Do the same calculation in reverse if your band is too loose: go down a band size, but go up a cup size.

Sometimes you may find a comfortable band width with a certain bra size, but the cups are a bit too tight. If you go up a traditional bra size to find a better-fitting cup, you might also experience a looser band. That's where bra sister sizes are helpful, because if you find the sister size to your established bra size, you may find a better-fitting cup — but you can still maintain the perfect band without it feeling too loose.

According to bra sister size rules, if your band fits but your cups don't, adjust the cup size. If it's too small, go a size up, and if it's too big, size down. However, if your cup fits but your band does not, then adjust both band and cup size. If you need to go up in band, go down in cup size, and vice versa. If neither your band nor your cup size fits, focus on adjusting the band size to your comfort and liking, and the cup size should naturally adjust.