Avoid stores that carry a limited range. A fitter at one of these shops might try to incorrectly sell you a size that they have on-hand, instead of your true size. Before you commit to a fitting, make sure the store carries smaller band sizes (such as 28 and 30) and larger cups (DDD and up). Good choices in the US include department stores like Nordstrom and Dillard's.
A fitted t-shirt will show up any bulges from cups which are too small, and likewise a moulded bra that is not filled out will show lines at the bust where the edge of the cups are visible. It's also useful to make sure that the colour of your bra is not showing through a thin or light coloured top - if you need to make your bra invisible, go for seamless cups which match your own skin colour rather than the colour of your top.
The fiber scientists in the Good Housekeeping Institute Textiles Lab are constantly testing undergarments like bras, pantyhose, shapewear, and swimwear both in the lab and with real consumer testers. The team evaluates things like washability, stretch recovery, fit, comfort, and more to find the ones that will feel great and actually last. We also got medical input on finding the best underwear for your health from Dr. Alyssa Dweck, MD, a gynecologist in New York and author of The Complete A to Z for Your V.
One of our staff testers had been wearing a 38B for years. So you can imagine her surprise when a professional fitter broke the news to her that she was actually a 36C. "Unbelievable!" the staffer exclaimed. "But you know what? The 36C feels much better — more supportive and comfortable." Her advice? Get thee to a professional fitter; nearly every department store has one. Or, if you're strapped for time, at least check your measurements yourself. Keep in mind, though, that bras are like shoes: No two fit the same. You have to try them on.