How big will I be with 400cc?

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.
Ultimately, careful selection of plastic surgeon and 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 "small D 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.
I hope this (and the attached link, dedicated to breast augmentation surgery concerns) helps. Best wishes. 

Hello and thank you for your question.  The
best advice you can receive is from an in-person consultation.  It is difficult to determine if a 400cc implant will be the right size for you without an in-person examination.  The size, profile, and shape of
the implant is based on your desired breast size/shape, your chest wall
measurements, and soft tissue quality.  This decision should
be based on a detailed discussion with equal input from both you
and your surgeon.  
Best wishes and good luck.
Richard G. Reish, M.D.
Harvard-trained plastic surgeon

Thank you for sharing your excellent question.  Because each brand of bra manufacturers sizes their garments differently it is impossible to guarantee a particular band size or cup size for patients prior to surgery.  Additionally there is no direct correlation between implant cc size and a bra size.  I would recommend seeing an ASPS board certified plastic surgeon for an in-person discussion.  They are in the best position to ensure that the implants the two of your decide upon are the right ones to meet your expectations.  It sounds as if it may be wise for you to "try on" several of the implants so that you can have a better visual representation as to what they would look like.  Best wishes!