"To find your band size, while keeping the tape snug, measure your underbust around your rib cage right where your bra band goes," Gillette says. Since bra bands come in even numbers, if you get an odd number, then go up to the following even number.  To measure your bust, put the measuring tape around the fullest part, once again, making sure that you keep it level. "To determine your cup size, subtract your underbust measurement from your bust measurement; each inch difference is a cup size," Gillette says. So, if you end up with a 3, you are a C cup.  "However, this starts to get tricky at 5 inches because, in US sizes, you would be a DD, while in European sizes, you would be an E, so if you're purchasing a full-bust bra, be sure to be mindful of the brand's size chart," she says.