Right now I'm in a c cup bra.. I'm wondering where I will end up in size with 400cc?

400 cc will not likely make you a DD.  It might not even make you a full D.  You best try sizers on to see what the volumes look like on your frame.  Only you can tell exactly what is going to make you happy.

A 400 cc implant should increase your cup size by 1.5 to 2 cup sizes. Being a C cup, that would put you at a full D to DD. Every patient is different, so a full consultation, sizers, measurements, and a 3D simulation will help determine the ideal implant for you.

No one can provide you with an accurate answer to your question. A 400cc implant will definitely make your breasts significantly larger.Generally adding 125cc will increase your bra size a by 1/2
to 1 cup size. Realize that there are no standardized cup dimensions of a
"C" or a "D". or a "DD".  It varies depending on chest wall size, bra
company and individual style within a company.Keep in mind that following the advice from a surgeon on this or any
other website who proposes to tell you 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 you find a plastic surgeon
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 FACSLa Jolla, California

Thank you for the question.  There is no accurate correlation between size of breast implant utilized during breast augmentation and resulting cup sizes. Ultimately, careful communication of your goals (in my practice I prefer the use of goal pictures and direct examination/communication in front of a full-length mirror) 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. I have found that the use of words such as  “natural” or "full 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. The use of computer imaging, may be very helpful during the communication process.
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).  Viewing the patient's chest wall in the upright and supine positions, with temporary sizes in place, help select  the best breast implant size/profile for the specific patient.
I hope this, and the attached link (dedicated to breast augmentation surgery concerns), helps. Best wishes.

I try to encourage my patients to focus on what they feel looks best on them rather than trying to achieve a particular bra cup size. Bras differ tremendously from one brand to another. Just compare a Victoria Secret bra to almost any other brand and you'll find a big difference. That said, a 400cc implant should give you a gain of about 1-1/2 cup sizes which should get you to a very full D or possible DD in the same brand bra you currently buy and wear. Remember, you'll always find a bra that fits. It's important to focus on the size implants that make you happy.

The answer depends on a great many features of your anatomy and can be answered after a consultation, not before, so see a surgeon and then you will know. If your surgeon is not guiding you as to the best size, you need to look elsewhere for your surgery.

For many reasons, it is not possible to predict how much a certain implant will increase bra cup size.  A very rough estimate is that 400 cc will increase your bra cup size by one to two cup sizes.  I recommend to my patients that they concentrate more on how a certain size implant looks than on the cup size.  If you tried on a 400 cc sizer and liked the look, then you are likely to like the result, independent of what bra size you wear afterward.  Hope this helps.