If I had a breast reduction with 400g removed & want old breast size back, can I get 400cc implants?

We can place a smaller than 400 cc implant because when they remove the breast tissue they removed the excess skin and now your breast tissue will be not expand enough. So smaller than 400 cc implant will be better choice.

Implant volume in ccs corresponds well to implant weight in grams. Therefore, if you had a breast reduction and are missing your old volume, adding an implant that is a similar size to the amount of breast volume that was removed is an excellent approach. It is also important to choose and implant that measures well for your frame, so this will also play a role when selecting the right implant to achieve your goals. In additional to choosing the right volume, you'll want to choose an implant that is the proper shape and measurements to achieve the results you desire.

I'm not a medical professional, but I can provide some general information.
In terms of breast size, it's essential to understand that the relationship between grams of tissue removed during a breast reduction and the volume of breast implants is not straightforward. The volume of breast tissue removed during a reduction doesn't directly correlate with the volume of breast implants needed to achieve a specific size or appearance.
Breast size is influenced by various factors, including the amount of breast tissue, glandular tissue, and fatty tissue, as well as the overall breast shape and chest dimensions.
When considering breast augmentation with implants, factors such as the implant size (measured in cubic centimeters, cc), shape, profile, and placement are all crucial considerations. Additionally, the existing breast tissue and skin elasticity will also affect the final outcome.
To determine the appropriate implant size to achieve your desired outcome, it's essential to have a thorough consultation with a board-certified plastic surgeon who can assess your individual anatomy, discuss your goals, and recommend the most suitable options for you. They can provide personalized recommendations based on your specific situation, taking into account factors beyond just the volume of tissue removed during your previous breast reduction surgery.
It's also important to have realistic expectations about the results of breast augmentation surgery and to understand that the final outcome may not be exactly the same as your pre-reduction breast size.
Ultimately, discussing your concerns and goals with a qualified plastic surgeon will be the best way to get personalized advice and guidance regarding your options for breast augmentation.