The cup size aspect of the standard bust measurement is calculated by the difference between two key measurements.
1. Chest measurement (around the part underneath the breast, where no tissue lies).
2. Cup measurement (around the fullest part of the bust, usually lined up around the nipples).
The cup is determined by the difference between these two figures. To clarfiy which cup size you are:
A difference of....
0-1/2 inch = AA
1/2 to 1in = A
1 to 2 1/2 in = B
2 1/2 in to 3 1/2 in = C
3 1/2 in to 4 1/2 in = D
4 1/2 in to 6 in = DD or E
6in to 7in = F
7in to 8in = G
continues with same pattern.
As an example, if I were to have a chest measurement of 32 inches, and a bust measurement of 38 inches, I would probably need to try cup sizes of 32D.
Remember that although you may be measured to be a certain size, bra styles and cups can vary from brand to brand so it may be worth selected your chosen bra in a cup size up and a cup size down when fitting. Indeed sometimes the shape will not flatter your chest so you may need a bigger size to be more comfortable.