Not all bras are created equal. Just because you’re a 34B in a demi bra doesn’t mean you’re a 34B in a full-coverage bra. “Think of it like jeans,” says Altman, “You might be a size 28 in boyfriend jeans, but you may need a bigger size in skinny jeans.” Like denim, different style bras have such specific fit and can vary by brand.
Demi bras are low-cut—sometimes called “half cups”—and they have significantly less material than a full-coverage bra in the same size. So you may wear a 34B in a demi bra but a 34A in a full-coverage, says Altman.
When in doubt, head to a specialty store for a bra fitting. “A good fitter will bring you a few sizes and a few styles so that you can really find what’s right for you,” says Altman. Wear a light T-shirt so that the fitter can get as accurate a number as possible when taking measurements.
“Would you wear a size 8.5 sneaker when you know that you’re an 8?” asks Altman. “Absolutely not.” Your boobs deserve to be treated just as well as your feet (if not better).