- Breast (and also bra size) is measured using band size and cup size.
To find your band size, move a tape measure around the ribcage directly under the bust. Add 5 to this measurement to obtain your band size.
For example: If your ribcage measurement is 29" + (5) = 34" band size. In case you get an odd number like 33", you can go to the next even band size (34").
To find your cup size, measure loosely around the fullest part of your bust i.e the tip of your breast. Subtract your band size from this measurement. If the difference is:
a) Less than 1 inch to 1 inch you are a cup A
b) Less than 2 inch to 2 inch you are cup B
c) Less than 3 inch to 3 inch you are cup C
d) Less than 4 inch to 4 inch you are cup D
e) Less than 5 inch to 5 inch you are cup DD
Hence, if your band size is 34" and your bust measurement is 35", you wear an "A" cup.
Additional information from brastraps.com