The A, B, C, and Ds Of Determining the Right Bra Size Right bra size. What do all those letters mean? Is a cup size 8 ounces? 1. Using a measuring tape, determine the number of inches around your chest, just below your breasts. 2. Add 5 inches if it is an odd number; add 4 inches if it is an even number. This is your bra measurement. 3. To find out what size cup you need, measure around the fullest part of your breasts. If the two numbers are the same, you need an AA-cup. If the two numbers differ by 1 inch, you need an A-cup. If the two numbers differ by 2 inches, you need a B-cup. If the two numbers differ by 3 inches, you need a C-cup. If the two numbers differ by 4 inches, you need a D-cup. If the two numbers differ by 5 inches, you need a DD-cup. Example Claudia's first measurement, around her chest, just below her breasts, was 28 inches. Adding 4 inches gives her 32 inches. This is her bra measurement. Claudia's second measurement, around the fullest part of her breasts was 34 inches. The difference between the two numbers (in this case, 32 and 34) is 2 inches, so she needs a B-cup. Her right bra size is 32B. Looking for the right bra size? Get helpful tips and advice from BeingGirl.