The process of how to determine bra size consists of two numbers:
Some manufacturers offer a bra cup size in half size options as well as regular sizes to help get the right fit. If you want to know how to measure your bra size, consult with a bra retailer. Most use a measuring tape to get accurate numbers from around your bust area. For added convenience, Playtex® has a downloadable tape measure on their site playtexbras.com.
Remember that by measuring bra size, you get the combination of the right cup and band size -- which is what you need to get a perfect fit.