First, we'll determine your FRAME Size.  The frame size is the diameter around your chest JUST BELOW your breasts. Using the tape measure, measure around your ribcage directly under your breasts. There should be no breast tissue measured while determining this number.

The next measurement goes around the chest, over and including the fullest part of your bust (usually at the level of the nipples). Technically, you should take this measurement while wearing a comfortable bra, though this is not mandatory. This is the diameter of your chest + breasts.