Bra Sister Sizes, What Are They?

Finding a well-fitting bra can seem like an eternal struggle — whether you’re trying out a new brand or just changing up styles from a label you trust and love. Knowing your measurements is key to getting an accurate size, but your particular breast shape may affect the way certain bras fit and feel. What matters most is that the style you ultimately choose is comfortable and gives you maximum support.

Here’s where sister sizing comes in. The concept is based on the grouping together of cup and band sizes that essentially offer the same fit and volume capacities, but with slight nuances that may be more compatible with your body’s unique proportions.

The general thinking is that as the cup size goes up, the band’s width must come down to maintain the ratio of how much breast tissue the cup will hold. Likewise, as the band measurement increases around the torso, the cup letter should decrease for the same reason. With sister sizing, a 34B would be equivalent to a 32C as well as a 36A.

Sound crazy? “There’s debate as to whether it is truly accurate — because while the cup size is the same, there are small but sometimes noticeable differences in fit due to the different band size,” explains True&Co CEO, Michelle Lam. “Our point of view: bra sizes differ anyway from style to style and even manufacturer to manufacturer. You are you. You are unique. So, consider it as another possibility to try.”

How to Use Bra Sister Sizes

Here’s a common scenario where knowing your sister size could help. You’re wearing your recommended bra size, but still, the fit issues can’t be ignored. Perhaps the underband feels too tight, even on the loosest setting, which is where you should always start. Or, it could be that the band’s too loose even on the tightest — possibly an indication that your bra has overstretched and it’s time to replace it. In the first case, you’d go up a band size as well as down a cup size, and in the second, you’d reverse it — downsizing your band while upscaling your cup. Of course, before exploring sister sizes as an alternative, be sure there’s no gaping or spillage from the cups. After all, the goal is to maintain the same volume and only adjust the underband’s tension in order to perfect your bra’s overall fit.

Find Your Sister Size

While it may seem complicated, finding your sister size is as simple as shifting the numbers and letters of your bra size one way each, in opposite directions. Say you’re wearing a 36C and while the cups do fit well, the band feels too tight. If you went up a number size to a 38C, you’d find that the cups were now actually too big — giving you the capacity closer to that of a D. Instead, the next size you’ll want to try is a 38B, which is the sister size that would deliver the right cup volume while also giving you a bit more give in the underband for added room and comfort.

At a glance, it may seem bizarre that a woman who’s always been an A could ever consider a D, or even a DDD, as a viable alternative. But, since the objective is to discover the bra that feels best on your body, broadening the fit spectrum to be more size-inclusive does make sense. By exploring sister sizing, it’s possible to find the best bra to fit your unique shape and provide optimal comfort within a variety of styles.

Here’s another potential application: the bra you want is sold out but there’s plenty of inventory left of your sister size — worth a shot to fit-test it, right? Since True&Co returns are always free and we’re all about helping you find the perfect bra, we say… give it a go!