It all comes down to sister sizing.

If you're someone who's wearing a bra right now, pause for a moment, and answer this question: Is your bra fitting correctly? Well, if you thought to yourself, "Yes, my bra fits well," sorry; you're probably wrong. "80% of women are wearing the wrong size bra, but 100% of women think they’re a part of that 20% wearing the right size,” jokes Laura Tempesta, the founder of Bravolution, who holds a master’s degree in lingerie design. There are many misconceptions about bra sizing, which is why so many wear the incorrect size. For example, if you wear a bra with an A cup, you probably think that means your breasts are smaller than someone who wears a D cup. This is actually false. Your breasts could be smaller, the same size, or even larger due to a concept called bra sister sizing. If you've never heard of this idea, that's completely OK. Tempesta explains exactly what sister sizing is to find a bra that fits your body seamlessly.

three bras on a line
Credit: HappyNati/Getty Images

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 32C, 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 under band,” 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 an antiquated system based on Victorian shirt sizing, which results in many women wearing a band size that’s too large, Tempesta says.

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—also known as 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.

Bra sister sizes reference chart
Credit: Illustration by Julia Bohan

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 to 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 skinny jeans that you are in boyfriend jeans, and sometimes there are styles that just don’t work for you,” Tempest points out. To that point, she says it’s always ideal for shopping at places that carry bras from a variety of different brands, including department stores such as Nordstrom and specialty 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.

Comments (1)

Better Homes & Gardens Member
June 22, 2021
This makes sense if you are a small person. No one has yet to develop a bra for plus size that is comfortable, pretty and fits right.