Bra Size Calculator - type in your measurements to get your band and cup size. Quote from the website: "Don't be alarmed if this size is different from what you are used to, most women are wearing the wrong size bra! Your recommended bra size is a great starting point to finding bras that will fit you perfectly. The best size for you may vary based on the bra brand, style or other factors."
This is one of the best sites for determining your bra size. Most people will be shocked by what it tells them. But then, 90% of women in the US are wearing the wrong bra size. Realistically speaking, you're wearing a sister size, like I am.