Until now, however, standard sizes have barely changed, although the range has expanded. Cup sizes are based on two measurements — the breast at its fullest point, minus the rib cage measurement. If it’s a one-inch difference, it’s an A cup; a two-inch difference, a B cup; and so forth. That approach, Jockey executives say, doesn’t account for different breast shapes.

Jockey began the project by scanning 800 women, getting “data points about all of the different measurements of a woman’s torso and the breast size,” said Sally Tomkins, a senior vice president. Researchers followed women in their homes as they chose bras and dressed, and heard “complete dissatisfaction about every aspect of the bra purchasing process, from the inaccuracy to the way you get measured,” said Dustin Cohn, the company’s chief marketing officer.

In the end, Jockey came up with 10 cup sizes. “Our bras don’t necessarily get bigger, bigger, bigger, but in different proportions — they get larger, but in different shapes,” Mr. Cohn said.

To fit the bras, Jockey uses a kit with 10 plastic cups in varying shapes, along with a measuring tape. Customers are meant to try on the cups and see what works best, then measure their rib cage. Someone with a 34-inch rib cage and medium-size breasts might wear a 5-34 or a 6-34, for instance.

Charla Welch, who reviewed the fitting process on her blog, The Bra Crusader, said the plastic-cup approach “wasn’t very comfortable.” “Maybe it’s just larger breasts, but I had to work it into the plastic cup,” she said. In standard sizes, Ms. Welch is a 32H, a 9-32 in Jockey’s size.

The Jockey Bra confronts several business challenges. First, the sizing kit costs online customers $19.95, although Jockey says it includes a $20 coupon, plus a money-back guarantee on a bra if it does not fit. Other companies that sell difficult-to-fit items, such as Warby Parker with eyeglasses, send customers try-on versions at no cost.