Bra Fit Mathematics

Drastic changes in cup size can come as a shock after a proper bra fitting. When you understand the math behind the measurements, it's much easier to accept.

Cups and Measures shared a clear and informative description she found on reddit in her blog.
http://cupsandmeasures.wordpress.com/2012/06/12/reddit-to-the-rescue-8/

"The letter is a measurement of the difference between the whole boob measurement and the around the ribcage under the boob measurement. So as the number (ribcage measurement) gets smaller, the same sized “boob measurement” gets larger, as the difference would be growing, giving you a higher cup size.So in this case, she thought she was 34C and found out that she was actually 32E, right? Her “boob measurement” (presumably 37″) is the exact same it was, meaning her boobs are actually no different, but the “ribcage measurement” is two inches less than she thought it was (she thought it was 34, but it was actually 32). This means that the difference between the two is actually 5″ different instead of 3″ which, according to the chart given to us by mywifesbusty, changes her assumed cup size from C to her actual cup size of E.The easiest way I can think of this is imagine huge boobs on a small woman. Then imagine the exact same boobs on a larger woman. The boobs seem to be more noticeably large on the little woman, giving her a bigger cup size out of the deal."

Be Your Own Bra Fitter

You already know how important it is to wear a well-fitting bra in the correct size. If you haven't gone to see a bra fitter and don't have plans to go any time soon, then consider taking a moment to measure yourself! You'll be glad that you did.
Good Housekeeping has a brief bra fitting guide here: http://www.goodhousekeeping.com/beauty/fashion/best-bra-guide-0800

Taking Your Measure
Not only will a properly fitted bra feel more comfortable, but it will make your clothes fit and look better. First step: Learn how to do the math, so you can figure out your own dimensions.
1. Stand straight and relax. Don't inhale and expand your rib cage as if you're about to blow out candles.
2. Using a soft tape measure, measure all the way around your body, placing the tape measure right beneath your breasts (example: 30 1/2). Add five to that number, rounding up if necessary to the next even number (36). Congratulations: That's your band size.
3. Next step: The cup. While wearing a bra, measure completely around the fullest part of your bust (across the nipple) without pulling tight. Make sure the tape measure goes around your entire back (38).
4. Subtract the number from your band size (38-36=2). The result will indicate your appropriate cup size, based on the following guide:
0=AA  1=A  2=B  3=C  4=D  5=DD  6=DDD

Read more: Bra Fitting Guide - Find the Right Bra Size - Good Housekeeping 

A Perfect Fit

No woman is average, and that is why True & Co is on a mission to create the perfect bra fit for women's unique figures. They studied the secrets of lingerie experts who are able to analyze the bra sizes of women at a glance and came up with a formula to determine the perfect bra fit! And it's all measuring tape-free.

See a bit of the work behind the algorithm this company is creating for making bras that fit the woman rather than the measured and predetermined definitions of bra sizes (as hazy as they are) here:
http://www.limelife.com/blog-entry/Forget-the-Measuring-Tape-A-Scientific-Formula-Now-Creates-The-Perfect-Bra-Fit/139483.html

"So what did they do? The pair began researching the untold secrets of lingerie experts around the country, many of whom never even use a measuring tape to determine a woman's bust size.
'We went to bra fitting after bra fitting and spoke to experts, and the best ones didn't use measuring tape," Lam said. "They had these unwritten rules where they could look at a woman’s body and say, we think this would be best. We were able to take these rules and distill them into code.'
All you have to do is fill out a short survey about your size and problems with your current bras, then make an order. Once you get your new bras, try them on and report back to True & Co. about how they fit. The site then recalculates your needs accordingly.
Want to try it out? Head over to the website now to get free shipping."