Summarize this article:

Each company will use a slightly different method to calculate bra size. If you can find a measurement guide or sizing chart for your preferred bra manufacturer, use that. You can alternatively receive a free fitting, over your clothes, at most department and lingerie stores. The method described below is a good starting point for figuring out your bra size but you may need to experiment to find the best fit. Keep in mind that bras may fit differently depending on their type. You may need a larger cup than normal, for example, with push-up bras. As with any clothing, it is also possible for bra sizes to run small or large as well. Measure your under bust using the method described in the clothing measurement section above. Add three inches to this measurement. If it is an even number then this is your band size. If it is an odd number, round down to the next even number to get your band size. Measure your bust at the nipple line as described in the monitoring weight section. The tape measure should just gentle touch you, not be pushing your breasts inward, and be parallel with the floor. If the resulting measurement is not a whole number, round up to the nearest full inch. This should give you a much smaller number (usually between 2-4). This number is used to calculate your cup size. Here is how the numbers correspond: 0-1/2 = AA 1/2-1 = A 2 = B 3 = C 4 = D 5 = DD This measurement system tends to be inaccurate for larger cup sizes and the system for your preferred brand should be followed.
Understand that there are many methods. Measure your under bust. Measure your bust size. Subtract your band size from your bust size.