The cup size is determined by this formula: Measure the the chest just below the breasts. Add the measurement of the breast at it's fullest point. For instance, if a lady measured 38" below the breasts and 42" at the fullest point, she would be a 38D.

Cup Sizes:

Up to 1" larger - A cup
Up to 2" larger - B cup
Up to 3" larger - C cup
Up to 4" larger - D cup
Up to 5" larger - DD cup
Up to 6" larger - F cup
Up to 7" larger - G cup
Up to 8" larger - H cup