Plastic Surgeons usually consider 1 cup size to be about 150 mls or cc. This means that when deciding on an implant size a 2 cup size increase is about 300 cc. When the implant is under the muscle you need to allow about 75-100 cc more. However none of this will help you when trying to decide what size of bra you need now or after surgery.
There are two important measurements to make. These measurements are in inches and apply mainly to the North American market.
The first is the band size around the chest. Use a tape-measure around the chest just under the breast. Keep the tape level on the front and back. Add 5" to this measurement to obtain your chest band size.
The second is the cup size. You should measure around the breast at the fullest part. The difference between this measurement and your chest band size is your cup size. Each inch of difference is a cup size.
Everyone is a little different so you may want to be measured by a professional.
You can get more information about measuring for a bra size at Victoria's Secret
Especially for larger sizes, some suppliers ( Bravissimo) are not convinced that measurement alone is the best way to get a good fit.
Breast Augmentation • Breast Reduction