Bra shopping is never as easy as we think it is, regardless of the size of your bust. While we initially look for something featuring frilly lace, or some soft satin or comfy cotton, shopping plans for something pretty can go awry once we decide to try everything on. Sure, the T-shirt bra might fit like a dream, but the balconette you grabbed in the same size doesn’t hold you in as well as you expected. And, you might not even be able to find the size you’re searching for on the shelves.
As frustrating as the shopping experience can be (whether online or in-store), there's a handy solution that many shoppers might not be aware of: sister sizing. It’s a system that offers a lot of leeway for those whose cups might runneth over, or whose cups might appear a little empty.
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Bras are one of the most complex garments to design, let alone fit such a large range of body sizes. According to Ra'el Cohen, head of design at ThirdLove, most of us might relate to the numbers and letters like they’re clothing sizes, but bra sizes are based on the ratio between two factors: the cup and the band. Because of this, manufacturers and retailers have a sister sizing system in which some bras have the same cup size but different band sizes, allowing for easy adjustments when in search of the perfect fit.
The easiest way to think of the system is as a grid, which is what you’ll likely find when you google “sister sizing.” According to Gill Heer, VP of design at Cacique, you can look up a retailer’s size chart to find your prospective sister sizes simply by following the diagonal line from your size and then do the math. For example, if your cups feel fine but your band feels too tight, you’ll want to go up in the band size (which sizes in two-inch increments) while also reducing the cup down a letter. On the other hand, if your band feels too loose, you can reduce the band size while bumping up a cup to make sure both fit comfortably.
“Another scenario where you’d need to try your sister size is if your band fits well, but your cup is either too big or too small,” Cohen said. “If your cup is digging into your breasts, you should size up in the cup (and down in the band) to find your sister size, and give your breasts some additional room.” Also, she says if your cup has extra room, you should size down in the cup and increase the band to find your best fit.
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“So if you are a 36C, your sister size is 34D or a 38B — and yes, if you took a measuring tape to measure the cup dimensions, it would be exactly the same across all three bras,” Michelle Lam, CEO and cofounder of True&Co., explained in an email.
That’s not to say designers don’t reach a limit. While the system will work for many, Heer says, “There are certain situations where you’ll get what we call a ‘size break’ where we feel that that mathematical grade doesn’t work beyond a certain size.” Larger cups, like F’s, G’s, and H’s, might need to adjust that sizing grade in order to accommodate them, unless the manufacturer chooses to stick to the mathematical grade and make a stop at a certain cup size.
Before we start browsing for your sister size, you’ll need to know your actual bra size to serve as your base. If it’s been six months since you’ve last had a proper fitting — we’re talking measuring tape and everything — experts say you should talk to a sales associate about checking your size, since breasts can fluctuate with the rest of the body, making it much harder to find your regular size or its sister.
Once you have your (new) number, you can start snagging a few styles you like, as this is where sister sizing truly shines. Remember our balconette vs. T-shirt bra debacle? While most might focus on the sheer size of the bra they’re trying on, the style you snap up can also have an effect on how well the bra fits you.
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Heer says that different body types may find a better fit with certain cup styles, which can shake up their subsequent bra sizes. “You can be the same cup size — mathematically the same cup size — but have either more or less breast tissue on top of your cup than there is [space for],” she says. Heer notes that younger breasts tend to have more flesh at the top of the cup, but the top becomes shallower as we get older.
And these different shapes may find a better home in certain bra styles than others, so the system makes it a breeze to find a great fit. For example, those who might have less flesh at the top of their cup in a more full-coverage style (regardless of overall breast size) can use the sister sizing system to find a well-fitting demi bra, which cover half to three-quarters of a full-coverage cup, or even a push-up style with foam in a size that might not be their go-to for other styles. If you’re spilling out of your cups, Heer recommends a style with a larger cup style, like a T-shirt bra, a full-coverage bra (with little to no foam or lining), or a balconette (if you want to show off a little more cleavage), and use your sister size to find the most accommodating cup for your needs.
If you’re unsure of the style that works best with your breasts, talk to a sales associate or hit that Live Help button if you’re shopping online to help you suss out the right style. And then if the cups don’t fit right in the dressing room, you can bump the cup a letter up or down and then reduce or increase the band size as needed. (Remember the ratio is inverted, so as one size goes up, the other must come down.) If you’re shopping online you can add your sister size to your cart and return the size that doesn’t work for you. Doing this will lead to a much better fit when you buy and ensure you’ll feel snug and secure, no matter the style that suits you.