First, using a seamstress tape, measure around your ribs (just below your bust) keeping the tape snug - not tight.
Next, add 4-inches to your measurement to determine your bra size. If your total is an odd number, then round up to the next inch because bras are only available in even numbered sizes.
Example; rib measurement equals 35-inches; plus 4-inches equals 39-inches, which is an odd number. Hence, round up to 40-inches. 40-inches will be your bra size. You can double check this number by measuring just above the chest from the center of the breast bone, under your arm to the center of your spine and multiplying that number by 2.
D-cup, 4.50"W X 5.50"H X 3.00"D E-cup, 5.50"W X 6.50"H X 4.00"D