The importance of a good fitting bra is often overlooked. The wrong sized bra can give the appearance of poor posture, as well as be very uncomfortable. Many women wear the wrong sized bra and don't even know it.
To get the best fit of a bra, put on your favorite, best fitting bra and take the following measurements:
Band size --
Using a soft tape-measure, measure around the ribcage directly under the bust. The tape should rest on the lower band of your bra. Be sure the tape is snug, smooth across the back and level with the front. It is often easier to have someone help you, so you can stand straight up. Mind your posture!
Add 5 to this measurement to obtain your band size. For example, if your ribcage measurement is 28" + (5) = 33" band size. If the resulting band size is an odd number like 33", you can usually go to the next even band size (34"), but you may also want to try the next lower band size (32") if you like a more snug fit.
Cup size --
Measure completely around the bust at the fullest point, holding the tape measure straight but not tight. The difference between this measurement and your BAND size is your cup size. Each inch of difference is equal to one cup size. For example, if your band size is 33" and your bust measurement is 36", you wear an "C" cup. Follow the chart below to determine your cup size.
If the difference is:
less than 1 inch = AA cup
1 inch = A cup
2 inches = B cup
3 inches = C cup
4 inches = D cup
5 inches = DD cup
It is also important when chosing a bra to determine its use. If you are going to be wearing it under a t-shirt, for instance, you will need one having fewer lines and a smooth texture so it doesn't show up underneath. You may also want to go with a padded bra, or one that has enhanced cups, so as not to be "revealing". However, if it is to be worn under an evening gown, you may want to go strapless. Think about your general wear and buy accordingly.