Not too loose, not too tight—here’s how to find a bra that fits you just right.

By Melanie Rud
Updated April 05, 2020

OK, let’s all be brutally honest with ourselves for a minute: Does your bra fit correctly? Really? “Eighty percent of women are wearing the wrong size bra, but 100 percent of women think they’re a part of that 20 percent wearing the right size,” jokes Laura Tempesta, founder of Bravolution and the only person in North America with a master’s degree in lingerie design (from De Montfort University in England). All kidding aside, it’s a common problem, one that stems back to misconceptions we’re taught to believe from when we start wearing bras, she says. Case in point, let’s play a game of true or false: If you wear a bra with an A cup that means your breasts are smaller than someone who wears a D cup. False. Your breasts may be smaller, the same size, or even larger. How, you ask? It all comes down to a concept known as bra sister sizing. Tempesta explains exactly what this is and how to use it to find a bra that fits you perfectly.

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Image courtesy of Getty.

What is bra sister sizing?

“Essentially this means that there are four different bra sizes—that all sound totally different—but all fit the same size breast,” Tempesta explains. For example, a 36A is the equivalent to a 34B, a 32 C, and 30D. Mind-blowing, right? Sister sizing is also why so many women are wearing the wrong size bra. “Most women are in the right volume or within the right sister sizing group, just wearing the wrong size underband,” Tempesta says. For example, you might be in a 34B when you should really be in a 32C. A band that’s too large is the most common problem; the way band size is measured is an antiquated system based on Victorian shirt sizing, which results in many women wearing a band size that’s too large, Tempesta says.

Related: Soma's Smart Bra Will Help You Find the Perfect Fit

How do you know if your band is too big?

The easiest way to do so is with a simple stretch test. Make sure your bra is hooked on the middle hook, then reach around and pull it back. If you can pull it back more than the width of your hand, you need to go down a band size, Tempesta says. (Or your bra may just be old and stretched out, in which case it’s time for a new one. In fact, while we’re on the topic, Tempesta advises replacing your bras annually.) If your band constantly rides up, that’s another sign that it is too big.

What about the cup size?

Once you’ve performed the band stretch test, take a look at the cups. If you’re in the right size cup, the small triangle of fabric above the underwire between your cups will lay flat against your sternum, Tempesta says. Double-busting—AKA the cups cutting into your breasts and creating an indentation—is another tell-tale sign, she says.  Looking at how much of your breast is showing above the cup is also helpful; if it’s the width of four or five of your fingers, your cup is too small.

Related: This Insanely Comfortable Bra Has Over 2,600 Near-Perfect Reviews on Nordstrom

Illustrated conversion chart for bra sister sizes.

So how do you find the right bra size?

While individual circumstances vary, there are two common fit issues that plague most women, Tempesta says. Scenario one: Your cups fit, but the band is too big. This is when you want go down a band size and up a cup size (sister sizing at its finest) from a 36B to a 34C or a 32C to a 30D. Scenario two: The cups are too small and the band is too big. In this case, your best bet is to stay in the same cup size, but drop a band size, for example, swap a 34B for a 32B. (This isn’t sister sizing because those two bras actually are different sizes, Tempesta says).

To set yourself up for shopping success, avoid the temptation to lock your (perceived) bra size in your mind, and simply grab a few different bras in that size. It’s imperative to try everything on, be armed with your sister size, and remember that not all bra styles and cuts are created equal. “It’s just like jeans. You probably aren’t wearing the same size in a skinny jean that you are in a boyfriend jean, and sometimes there are styles that just don’t work for you,” Tempest points out. To that point, she says it’s always ideal to shop at places that carry bras from a variety of different brands—department stores such as Nordstrom, lingerie stores—versus stores carrying only their own line. Now that you have these expert sister sizing tips and our bra size conversion chart, you'll never have to second-guess your fit again.

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