What will my bra size be with 450cc implants? My rib cage is 27 In and around my breasts is 31.

Thank you for the question.   
There is no accurate/reliable/predictable correlation between size/profile breast implant utilized and cup size achieved. For example, I do not think that the generalizations of "200 cc equates to 1 cup" hold any validity or are helpful in any specific or practical way.  In my practice, I ask patients not to communicate their goals, or evaluate the outcome of the procedure performed, based on cup size references.
Generally speaking, every patient undergoing breast augmentation surgery should understand that their anatomic starting point will limit the outcome of breast augmentation surgery.  Other factors involved include careful breast implant pocket dissection and careful selection of breast implant size/profile.   
The use of a larger breast implant (than what is safely "allowed" by the patient's anatomy) can be problematic;  potential problems include breast implant displacement/malpostion issues ( such as bottoming out, lateral displacement, symmastia...)  and/or significant breast implant rippling/palpability.
Ultimately, careful selection of plastic surgeon and  careful communication of your goals (in my practice I prefer the use of goal pictures, direct examination/communication in front of a full-length mirror, in bra sizers, and computer imaging) as well as careful measurements (dimensional planning) will be critical.
  Generally speaking, the best online advice I can give to ladies who are considering breast augmentation surgery ( regarding breast implant size/profile selection) is:
1. Concentrate on choosing your plastic surgeon carefully. Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you are looking for. Ask to see lots of examples of his/her work.
2. Have a full discussion and communication regarding your desired goals with your plastic surgeon. This communication will be critical in determining breast implant size/type/profile will most likely help achieve your goals. 
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. For example, I have found that the use of words such as “natural” or "DD cup" etc means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on him who makes the bra; therefore, discussing desired cup size may also be inaccurate. Again, the use of computer imaging has been very helpful during the communication process, in our practice.
3. Once you feel you have communicated your goals clearly, allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals. Again, in my practice, this decision is usually made during surgery, after the use of temporary intraoperative sizers.
In your case, my best advice: more communication. It will definitely worth spending more time (prior to the date of surgery) with your plastic surgeon to discuss further.  As you know, it will be important for many reasons, that you feel comfortable that you and your plastic surgeon are on the "same page" prior to proceeding.
I hope this helps. Best wishes for an outcome that you will be very pleased with.

If your chest is extremly small, the cup size does tend to increase disproportionatly as the breast size is increased. If you are currenty wearing a 30C , you could very easily require a DD or larger if a large implant is placed. Sometimes going to a larger chest size, such as 32, will allow you to have a smaller cup, such as a D instead of a DD. I recommend making your breasts look the way you want, and not worrying too much about actual size.

The best cosmetic result in any particular breast augmentation patient depends on a variety of factors, including: Your individual anatomy, desired outcome, realistic expectations, a thorough discussion with the plastic surgeon about the options, and an understanding of the pros and cons of any particular implant choice. Proper sizing is not just about the number of cc’s. The thickness of your tissue, breast dimensions which include the width, height, and projection, as well as chest wall width all need to be considered when choosing an implant. Trying on implant “sizers” of various shapes and volumes while wearing a tight t-shirt, bra, or bathing sit at a preoperative visit will help you and your surgeon choose the optimal implant. 

There are no manufacturers' standards for cup sizing in the bra industry. The cups of a 30 D-DD+ and a 36 D-DD+ are significantly different. Cup size varies from manufacturer to manufacturer and even within styles from any particular manufacturer. Nor is there a direct correlation between implant shape or size and cup size. 

Keep in mind that following the advice from a surgeon on this or any other website who proposes to tell you exactly what to do without examining you, physically feeling the tissue, assessing your desired outcome, taking a full medical history, and discussing the pros and cons of each operative procedure would not be in your best interest. I would suggest that your plastic surgeon be certified by the American Board of Plastic Surgery and ideally a member of the American Society for Aesthetic Plastic Surgery (ASAPS) that you trust and are comfortable with. You should discuss your concerns with that surgeon in person.

Robert Singer, MD FACS 

La Jolla, California