People are learning their real bra sizes thanks to a calculator that's gone viral on TikTok (2024)

This viral breast size calculator that blew up on TikTok may change the way you think about bra sizes entirely.

People began discovering they'd been wearing the wrong bra size in early April, when lingerie model turned lingerie educator Madison Alexandra posted a video showing viewers how two very differently shaped breasts could actually be the same size. That's because cup size is measured by volume, not visual projection. A breast that is rooted to the chest wall over a smaller surface area, the creator explained, is going to seem more projected from the torso than breast tissue rooted over a larger one. In other words, breasts that take up less lateral space can appear larger than breasts that are spread across a wider space. Though they may be the same size, the wider-set breasts will appear to be more shallow.

She demonstrated this using a sock filled with rice; even though the amount of rice in the sock didn't change, reshaping it to be wider and flatter made it appear vastly different from when it was more narrow.

This was a revelation for most TikTok users, many of whom had been sized according to outdated methods.

CharisseV, another TikTok creator, posted a response video explaining that although she usually wore an A or B cup, the underwire always seemed to dig into her actual breast tissue. In the video she posted, which now has 12.3 million views, Charisse measured herself using A Bra That Fit's calculator and found that she was actually a 32D, which she initially didn't believe.

"Let's try this on, there's no way," Charisse said, after a trip to Victoria's Secret.

Charisse was shocked when the bra actually did fit her, and that it fit more comfortably than her previous size did. Upon resizing herself, she found that a 32DD fit better, and compared the 32DD bra to the 34B bra in a follow up video. Though they appeared to protrude the same amount, the properly fitting bra was wider in the cups and more narrow in the band.

Most American retailers and lingerie brands size bras by measuring the underbust, adding four inches (five if the underbust measurement is an odd number) and subtracting the total from the bust measurement. The measurement around your underbust — the area around your ribcage where your breast tissue ends — and the additional four inches is supposedly your band size, and the difference between that number and the measurement around where your breasts are the most projected from your body is your cup size. A one-inch difference is an A cup, two inches is a B cup, three is a C cup, four is a D cup, and so on.

But that "plus four" method — an antiquated practice tracing back to the 1950s when bras were manufactured with fabric like silk and satin that didn't stretch, doesn't make sense with modern day textiles. Even though today's bras are made with elastic fabric to account for breathing room, most retailers still size bras using the "plus four" method.

Under these sizing guidelines, someone with a 32-inch underbust and a 36-inch bust measurement would be sized as a 36A. And while the cup size may fit, they'd receive support from the bra's straps, rather than the band, which can put additional strain on their shoulders and make wearing bras uncomfortable. Without using the "plus four" method, that person could be a 32D, which has a similar cup volume but much more tightly fitting band. That provides support via the band, rather than the straps, which can alleviate some of the neck and shoulder pain people experience while wearing bras.

According to lingerie expert Cora Harrington, who runs the site The Lingerie Addict, some brands do still require wearers to size themselves using this method. In a 2012 blog post about the "War on Plus Four," a campaign spearheaded by UK-based lingerie blog Busts4Justice, Harrington switching out one formula for another doesn't solve the core issue that many bra wearers face: not knowing how a bra should fit. The "plus four" method doesn't work for everyone, but taking the underbust measurement as-is may not work for everyone, either because of differences in breast shape, muscle definition, and support needed. What's more important is understanding how a well-fitted bra sits on your body, and figuring out what works for you from there.

"If a woman knows how the band is supposed to fit and how the cups are supposed to fit and how the gore is supposed to fit and so forth and so on," Harrington wrote. "It won't matter if they're far away from a boutique or if their bra size changes or what brand they buy from because then the core knowledge will be there."

In addition to wearing the wrong size, most bra wearers don't "swoop and scoop" their breasts into the bra's cups, which molds the breast tissue into shape rather than allowing the bra to "sit" on top of it. That prevents uncomfortable underwire digging, and ensures the wearer "fills" the cup.

Because most people don't know their true size, there's little demand for extended sizing, and retailers continue to stock a limited scale of sizes. With few options for extended sizing, customers often buy bras based on what they can fit into, rather than what fits them. Retailer Curvy Kate, which specializes in extended sizing, describes this as a "vicious circle" in which brands aren't motivated to manufacture more sizes, and customers are sized incorrectly to fit into whatever retailers have in stock.

The subreddit r/ABraThatFits is a longtime advocate for leaving the "plus four" method in the past. While the community has been around since 2010, it's now receiving a new wave of attention thanks to TikTok.

Since Charisse posted her video, thousands of other people have used the calculator to figure out their updated bra size. Most, like Charisse, found that they've been wearing a larger band than they need to, with a smaller cup, so bras never quite fit comfortably. The tag #abrathatfits racked up 32.4 million views on TikTok as of Thursday, and the online calculator received so much traffic that it crashed for two days.

"I'm so glad the calculator is getting the recognition it deserves," Madison Alexandra told Mashable in an Instagram DM. "I've been doing bra fitting educational videos on TikTok this year trying to get the message out there that most women are suffering unnecessarily from poorly fitting bras due to the lack of knowledge about how bras should fit...that calculator was a life changer for me about three years back in starting my bra journey and I've been trying to pump it up ever since!"

This particular calculator is unique because it relies six measurements, as opposed to the traditional two, to determine your bra size. It asks for a tight, snug, and loose underbust measurements, as well as bust measurements while standing up straight, bending over, and laying down. This accounts for the fact that breast tissue shifts with gravity, but the volume stays the same.

It also takes the user's assigned gender at birth into account, since breast tissue develops and sits differently on various ribcage shapes. Many trans feminine and trans masculine people have turned to A Bra That Fits for advice on finding the right bras after hormone therapy or gender-affirming surgery, as retailers' sizing charts are usually designed for a very specific subset of cis women's bodies. In a TikTok, the calculator's creator, Freda, explained that AMAB (assigned male at birth) people tend to prefer a looser band than AFAB (assigned female at birth) people, because of ribcage and breast shape differences.

The calculator also offers suggestions for "sister sizes," as some people prefer a looser band or have trouble finding retailers that stock their size. Once you've figured out your size, you can use A Bra That Fits' guides to determine your breast shape, which can help you find bras that fit more comfortably.

Since going viral, the subreddit's moderators have created a TikTok account and a handy starting guide to figuring out sizing. If you're shocked by the size the calculator comes up with, the moderators also run an online gallery of user-submitted images that shows the multitude of ways breasts can look across sizes.

At the end of the day, figuring out bra size should be centered around what fits you, rather than what you happen to fit into. Luckily, the A Bra That Fits community can offer support when your own bras can't.

UPDATE: May 2, 2021, 11:08 a.m. PDT Updated with comment from Cora Harrington and Madison Alexandra.