Bras are fitted with two measurements: the circumference of the bust and the circumference of the ribs immediately below the bust. To take this measurement yourself, always make sure your measuring tape is parallel to the ground (you may wish to use a mirror). Or you can have a friend help you. Here's how to take this measurement:
Wear a bra that fits you well, but not a padded one or it will throw off your measurements. Next, measure around the fullest part of your bust. This is going to be right about where your nipples are. Be sure not to do any binding with the measuring tape. You need to have a measurement of your bust size as it is naturally. This is your bust measurement.
Now you can take your band size measurement. Breathe out as much air as you can so that this measurement is as small as possible. Then, using your measuring tape, find the circumference of your ribs. If it is an even number, add 4 inches to it. If the result is an odd number, add five inches. Now you can find your cup size.
Take your bust measurement and subtract your newly summed band size measurement from it. For every inch difference there is, add a cup size. So, one inch difference means an A cup, two inches means a B cup, three inches means a C cup, and so on.
If you are an unusual size (very, very large-busted), this system may not work as well for you as going to a professional and getting measured and fitted by that person. You can find one of these people in any upscale bra boutique, as well as in lingerie stores. However, don't feel pressured to buy anything just because you got measured there.
Now, signs you need a new, better-fitting bra: