I have a lingerie magazine handy and this is what they say....
First measure under the bustline. Put on your best-fitting, unpadded, underwire bra. Measure underneath the bustline and make sure to measure tightly. Also, make sure the tape is straight across the back.
If you come up with a half measurement, like 32 1/2, round UP to 33". If you measure between 32 and 32 1/2, round DOWN. (In general, the rule of thumb for all measuring is: less than 1/2", round DOWN, more than a 1/2", round UP.) (write it down, Measurement 1)
After that, figure out your band size. If measurement 1 (the bustline) is
under 33 inches, add 5 inches. If this number is odd, round up to the next even number. If measurement 1 is
over 33 inches, add 3 inches. If this number is odd, round up to the next even number. (write it down, this number is your Band Size)
Next, measure over the biggest point of your bustline. Make sure the tape is straight, and dont pull it too tightly. (write this number down as well, Measurement 2)
Lastly, calculate your cup size. First, subtract measurement 1 from measurement 2. Then consult the following chart to find your cup size.
If measurement 2 is......
0" larger than measurement 1, then your cup size is AA
1" larger than measurement 1, then your cup size is A
2" larger than measurement 1, then your cup size is B
3" larger than measurement 1, then your cup size is C
4" larger than measurement 1, then your cup size is D
5" larger than measurement 1, then your cup size is DD
6" larger than measurement 1, then your cup size is F
Then obviously, combine your band size and your cup size to get your bra size