If I wear a size 40N bra and had a reduction to a D or DD cup size. How much tissue would be removed?

At
size 40 each 300 grams removed corresponds to 1 cup size change.If your measurement is accurate, you would
require 2,700 grams removed from each breast to achieve a DD.The largest breast I have ever performed was
a 44 M and I did remove approximately that amount to achieve her goal of a 42
DD.Best Wishes,Gary Horndeski, M.D.

 Congratulations on your decision to proceed with breast reduction surgery. You are correct in that you will be best off achieving a long-term stable weight prior to proceeding. They will be well worth your effort; breast reduction surgery tends to be one of the most patient pleasing operations we perform.  Well experienced board-certified plastic surgeons will be able to give you a much better “estimate” in person than with online consultation.  You will find that there is no accurate/reliable correlation between amount of tissue removed and specific cup sizes achieved. Remember, as you communicate your goals with your plastic surgeon, that a discussion of cup size can be confusing and/or a source of miscommunication. In my practice, I prefer the use of goal pictures to help with this communication process. Generally speaking, I suggest patients undergoing breast reduction surgery have enough tissue removed to help alleviate/decrease  symptoms they are experiencing,  while preserving enough tissue to feel comfortable about the upper and lower body  proportionality.
Best wishes.

There's no way to tell you how much weight needs to be removed to be a certain cup size. The surgery is not dictated by cup size. It Is dictated by shaped volume and proportion to the rest of your body.