The following has been borrowed with permission from www.implantinfo.com
For those of you seriously contemplanting breast augmentation, this is a great site for
research, information, participation in chat rooms and for review of before/after photos.
DETERMINING YOUR CUP SIZE
For no apparent reason, finding out your bra size is always a big mystery.  It shouldn't be.  
Although there is no foolproof system, the following set of guidelines should get you most
of the way there.

Your bra size is determined by both a band size (e.g., 36) and a cup size (e.g., C) to come
up with the bra size (e.g., 36-C).  There are several ways to measure both elements.  We
are going to do all of them and use a combination of the results to come up with your best
estimate at size.
1.    Measure your "band" size.   
We are going to arrive at this by using two separate measurements.   First,
measure around your upper chest, that is, right under your armpits and above
your breasts, as shown in Figure 1.

In Figure 1, the measurement is just over 35".  According to many bra
manufacturers' instructions, you take this measurement and discard any
fraction. In our Figure 1 example, 35.25 inches becomes just 35 inches. If the
result is an odd number, you will round up to the next even number, in this
case a 36.  You now have one possible measurement of  your band size.
2.    Alternative Band Size.  
Alternatively, you can measure your band size by measuring the ribcage just
below your breasts, as in Figure 2.

In Figure 2, the measurement is about 31.5 inches.  Again, discard any
fraction, leaving us with 31".  Add 5" to the measurement if your ribcage is odd
and 4" if it is an even number.  In our example, we are also at a 36" band size.  
So far so good.
3.    Measure cup size.   
Traditionally, you would take one of your measurements above, and subtract it
from the measurement of your "bust", as measured in Figure 3.

In Figure 3, the measurement is 37.5".   Without discarding any fractions, take
37.5" and subtract the measurement you obtained in step one (35.25").   The
resulting number (in this case 2.25"), is supposed to dictate cup size.  You
round up from any fraction.   The number of the difference, rounded up, is your
cup size.  A cup = 1 inch difference, B cup = 2 inch, C cup = 3 inch, D cup =
4 inch, DD cup = 5 inch and so on.   In my Figure 3 example, the 2.25"
rounded to 3" results in a C cup.    For the record all three steps are completely
accurate on the model featured here, so far.
4.    Alternative cup size measurement.  
Thanks to Ed Pechter, M.D., author of The Right Bra Measurement system,
there is a new way to measure Cup Size.   For many women, step three is
simply not accurate.  Women with very large or very petite frames, "unusual
breast shapes, etc. complain that the traditional measurement does not work.

As in Figure 4, measure across the fullest part of the breast (across the nipple),
starting at the outside of the breast crease and going to the inside of the
crease.  Figure 4 shows the measurement of 9 inches.

The Right Bra measuring system allocates a cup size to each measurement.  
7" = A cup; 8" = B cup;   9" = C cup; 10" = D cup ; 11" = DD cup; 12" = DDD (E
cup).
Copyright MD Aesthetics, LLC. 12/04
*** “The Right Bra” cup size formula in step 4 is most accurate for women with band sizes in the 34-36” range.   
Because manufacturers make cup sizes smaller for smaller band sizes and larger for larger band sizes, women with a:
30-32" band size should deduct about one inch from these measurements (A cup = 6", B cup = 7" and so on).  
38-40" band size will find that bras are upsized in cup size (A cup = 8", B cup = 9" and so on).

*** No system is foolproof.  Because bands that are too tight are just not comfortable, use the larger of the two
measurements obtained in steps 1 and 2 for band size.  Also, use the larger measurement in cup size from steps 3
and 4, depending on manufacturer.  



ADDITIONAL RESOURCES FOR DETERMINING CUP SIZE OR RESULTS FOLLOWING BREAST AUGMENTATION:
Quick LInks:

www.lookingyourbest.com/inamodel/index.html

www.justbreastimplants.com/size.html

If the above links are not active, please cut and paste addresses to your browser.
Cup Size
Determining Your Size (Bra/Cup)
Nicole's Measuring System
modified from
www.implantinfo.com
The preceding has been borrowed with permission from www.implantinfo.com