I'm 21 years old, 5'5, 107lbs and a 32A. Will 400cc Silicone implants give me a full C cup?

400 cc's is in the right size range. It will probably make you a large C.  Any larger you will probably be into the D range.

200 cc is about one cup size in an average size woman, as a gross estimate.  Therefore, 2 cup sizes would be about 400 cc.  The best way to determine what you would like is to try on sizers in the office.Kenneth Hughes, MDLos Angeles, CA

Choosing the right implant to meet your goals can be a tough decision.  I wouldn't get too caught up with an exact "cc" to reach a desired cup size, because the implant size required will depend on your specific anatomy.  This includes the width/height of your chest wall (rib cage), and the position of your breasts.  A good surgeon should be able to look at your chest, and identify the range of implants that would achieve your desired shape and size.  In my practice, patients bring photos to their pre-op appointment that demonstrate the size and proportion of the breasts they desire.  This eliminates the ambiguity of cup size since a Victoria's Secret C-cup is different than a Calvin Klein or Nordstrom C-cup bra.  In the operating room, I use temporary sizing implants to match your breasts to the pictures you have picked, and can then select the exact implant needed from a large consignment I keep in the OR.  This has been an excellent method to match a patient's desired size, and foregoes the need of selecting your specific implant prior to surgery.

Hi. To answer this question I would need to see photos of your chest/breasts including breast width measurements. Different people can achieve different results and cup size with the same implants - it depends on your chest wall size, breast size/position before the surgery as to what cup size and whether you get true cleavage with a particular implant.
The best things is to ask your surgeon to show you photos with similar body
frame and similar size implants to the ones you are thinking of to get a
realisitic idea of what to expect.
Try to find someone with a Vectra 3D imaging machine in your area to simulate
your breast augmenation result (see the link below) - in a recent article (Oct
2013) of Plastic& Reconstructive Surgery Journal (Plast. Reconstr. Surg.132:
810, 2013.), the main Plastic Surgery Journal worldwide, the 3D imaging VECTRA
machine was found to be beneficial for patients considering breast augmentation
surgery.

The article concluded that patients prefer a center that offers
three-dimensional imaging technology; they feel that the simulation is very
accurate and helps them greatly in choosing the implant; and that if they could
go back in time, they would choose the same implant again.

All the best with your decision. Regards Dr Charles Cope

It's difficult to say without an exam or photos but generally speaking 150-200cc is about a cup size.  It would be best to see a board certified plastic surgeon for a consultation and exam to determine what implant would be the best for you.  You could also try them on at that time, which I find is the best way to help with the decision process. ac

and if you put 1.5 cups of rice in a nylon stocking and 'stuffed' your bra, you would get a very good idea of what a 400 cc implant would look like on your frame.  But the last thing I would do is allow the doctor to choose for you... its your body and you should be making the final decision as to what you will have.

Thank you for the question.To best help you, a physical examination or picture evaluation are required in order to determine what size of implants would be best for your case.Dr. Campos

Thank you for the question. Unfortunately, online consultants will not be able to provide you with specific enough advice to be truly helpful. Given your questions and concerns, you'll be best off spending additional time with your plastic surgeon communicating your goals carefully. Keep in mind, that there is no reliable correlation between the breast implant size/profile use and specific cup size achieved.
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're 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 C 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 technology may also be very helpful during this 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.
I hope this (and the attached link) help.

The best cosmetic
result in any particular breast augmentation patient depends on a variety of
factors, including: your individual anatomy, 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. Currently, FDA regulations restrict the use of silicone gel filled implants to patients 22 years of age or older. Saline filled implants are available for use in a 21 year old.

There are no manufacturers' standards for cup sizing in the bra industry. The
cups of a 32 C and a 38 C are significantly different. Cup size varies from
manufacturer to manufacturer and even within styles from any particular
manufacturer.

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 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 FACS

La Jolla, California

Good question but I would stop thinking about cup size and focus on how large you want to be.  Everyone is different and what works for one does not always work for the next because of breast shape and body size. I would see your PS and try on sizers to see if you really want to be that big.  Remember that your surgeon and only work with the possible and not always the wanted.  He or she will need enlarge what you have and if you have gap before you might have one after.  Listen to your plastic surgeon follow their advise.