Katie Williams tries to wait until she gets home from work, but sometimes she doesn’t quite make it. One hand on the steering wheel, she reaches around her back, finagling the clasp until the cursed contraption of fabric and wire finally comes loose.

At last: freedom.

Williams, 22, can’t remember a time when she wasn’t bothered by her bra. The straps were too tight, the band was too loose, the underwire dug into her armpit. “Honestly,” she said, “I’ve never really felt like I’ve ever worn a bra that fits.”

In late April, when Williams watched a TikTok video about a new bra-sizing method, she said, she was intrigued. She got out her measuring tape and recorded her breast size in each of the specified positions: standing up, bending over, lying down. She was a 38 I, according to the calculator. All her life, she’d been wearing a 36 DD.

Approximately 80 percent of people wear the wrong bra size. Since Oprah declared a “bra revolution” on her show in 2005, popularizing that statistic, bra manufacturers and major retailers have been trying to capitalize on the discomfort, said Kiersten Ballman, who has been sizing clients at Coup de Foudre Lingerie in D.C. for over 10 years. Companies promise women the perfect fit, but bra sizing is complicated, she said — even a department store fitting might lead you to the wrong size. Frustrated by the aura of mystery around the process, several women have taken to social media, creating viral videos that have helped many to uncover their true size on their own.

Earlier this year, Madison Alexandra McFarlane went to every store in her mall that offered bra fittings, filming her experience for TikTok.

“They gave me like 10 different sizes,” she said.

McFarlane, a lingerie educator, went to the mall that day to confirm for her viewers what she already knew: Professional bra fittings are not always accurate. When she started gaining weight after an eating disorder, McFarlane said, she realized she needed a new bra size. She went to a few stores and shopped around online, she said — but nothing felt comfortable. Eventually she sought out advice at a boutique lingerie store, where she was told she was a “28 L.”

“I was like, ‘I’ve never heard of this size before. Most stores start at a 32 band, and go up to a DDD cup,’” she said. “How is this even possible?”

There are two components to a standard bra size: A number, representing the size of the band, and a letter or series of letters, representing the size of the cup. Most American bra retailers carry sizes that range from a 32 to a 38 band, and from an A to a DDD cup, a significantly smaller range than what’s on offer in Europe, McFarlane said.

American retailers will “put you in something that looks like it fits,” McFarlane said. A U.S. department store would never say she is a 28 band or an L cup, she said, because they don’t have either of those sizes in stock.

Even if you fall within the standard American range, the perfect bra size might still be hard to nail down. Bra sizing is “counterintuitive,” said Ballman, the lingerie store owner. When people come into the store wearing the wrong size, she said, it’s often because they have a band that’s too big and a cup that’s too small. If you’re sizing yourself, or being sized by someone without proper training, you’ll probably go for what feels comfortable in the moment, she said, opting for a looser band because the bra feels tight. But while the looser band might feel more comfortable initially, Ballman said, it leaves your breasts unsupported.

“Ninety percent of the support in your bra is in our band. It should feel like a comfortable hug,” she said.

Many of the recent TikTok videos reference a bra size calculator that accounts for this problem. The calculator is “super helpful” for women who are wearing too big a band and too small a cup, especially if they require a size not typically carried in U.S. department stores, said Ballman, who watched one of McFarlane’s recent videos and took a look at the calculator. Retailers will often increase band size to compensate for a larger cup size they don’t carry, she said.

Once you find your bra size, you’re still not done, Ballman said. Sizes vary significantly between brands, styles and materials. Even color can impact a cup size, she said.

“I could have nude bra and a black bra: same style, same cup, same brand. But the black dye saturates the fibers and makes for a stiffer fit.” In the black, Ballman said, her clients might have to go one cup size up.

The bra sizing process is “subjective,” based on dozens of factors that are hard to weigh simultaneously, she said. Along with their size, Ballman said, she considers what her clients prioritize in a bra: Do they care more about comfort or silhouette? It’s also important to remember that bra size changes over time, she said, impacted by weight loss or gain, pregnancy, breastfeeding, aging, and hormonal shifts.

Once you find it, the right bra fit can be transformational. After posting on TikTok, McFarlane heard from dozens of women who said they had never felt confident about their breasts: They were too big, too small, too flat. “So many women hate on themselves, thinking they’re never going to look good in a bra.”

Then they try on the right size.

In her store, Ballman sometimes sizes women in their 50s and 60s who have been wearing the wrong size their entire lives. “I get tears,” she said. “They’ll tell me, ‘I feel like a new person.’”

In an ideal world, everyone would get sized by a trained professional, McFarlane said. But the vast majority of women don’t have access to that kind of expertise. Online calculators are the “next best thing,” she said, especially when used in conjunction with bra-fitting forums. The popular subreddit “A Bra That Fits” offers shape guides and crowdsourced bra-fitting consultations, with thousands of people weighing in on how sizes fluctuate from brand to brand. The forum is catering to the new wave of bra comfort seekers: A post at the top of the subreddit reads, “If you’re from TikTok, start here.”

When Fae Henry, 21, used the TikTok bra calculator, they were shocked. They had been wearing either a 32 D or a 34 C. The calculator told them they were a 32 or 34 DDD.

Henry decided to give it a try: They struggled to keep their breasts in place while they were kickboxing, constantly adjusting their bra as they worked out. With the new bra, they said, they didn’t have to worry.

“It’s the best fitting bra I have ever owned.” As soon as Henry can save up the money, they said, they plan to buy more.

Bras can be extremely expensive, especially if the size falls outside the standard American range. A custom European bra can cost as much as $200, McFarlane said. She sees her bra drawer as an investment, she says, including a line for bras in her annual budget. Bras are highly technical support garments, McFarlane said, so pricey because they can include up to 25 distinct components, more than other articles of clothing.

To bring down the price of custom bras, in particular, U.S. retailers need to carry more bra sizes, McFarlane said. In the meantime, she added, if you’re ordering on Amazon — which offers some of the most affordable bra brands — be prepared to send some back. There will be a lot of trial and error.

After she finished the calculator, Williams scoured Amazon until she found a bra in size 38 I. When the bra arrived, it didn’t quite fit.

Williams was disappointed, she said, but will not be deterred.

The TikToks have inspired her to keep trying — until she finds a bra she doesn’t need to unclasp in the car.

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