In bra fitting, the numbers (8, 10, 12, 14, 16, 18, 20) refer to your body size and will usually be the same as the normal clothing size that you wear. The letters (A, B, C, D, DD, E, F, FF, G) refer to your breast size. A lot of people think that a 12D and a 16D will have the same size cup and just a different body size band. It is important to note that as the Body Band size increases, the cup size decreases in proportion.
So if you need to go up a Body Band Size, then you need to go down a cup size in order for the cup to have the same fit.
For example, a lady finds that a bra size 12DD the band is too tight. Solution: she needs to try a 14 Body Band size. In order to have the same size cup, she will need to drop a cup size to a D. She will need to try a 14D.
Another lady finds that a size 16E is too large and she wants a smaller Band Size. She drops a Body Band size down to 14, and increases cup size to F. She will need to try a 14F.