Bra size is determined by taking two measurements around the bust to determine band size and cup size. The combination of band size and cup size indicates the proper bra to purchase.
To measure band size, stand in front of a mirror and wrap a flexible tape measure under the arms and around the bust. Keep the tape measure at the center of the back and the upper portion of the breast, where the bra straps meet the cup. If the measurement results in an odd number, round down to the nearest even number.
To determine cup size, use the flexible tape measure to measure under the arms and around the fullest part of the bust, making sure that breathing is still comfortable. Subtract the band size from the cup measurement, and use the difference to determine cup size.
For each inch of difference between band size and the cup measurement, add one cup size. For example, a 36-inch cup measurement and a 34-inch band measurement result in a difference of 2 inches and therefore a cup size of B. For this set of measurements, the complete bra size is 34B. If the difference between band size and the cup measurement is less than 1 inch, then the cup size is AA.
If still in doubt about your proper bra size, go to a local lingerie store and ask to be professionally measured.