It is easy to determine the bra size, when you follow the steps. First, you measure around your ribcage under the breasts using a paper tape measures in inch. The bra size is the number you measured plus 5. Now you have to determine cup size of the bra. Now, you measure around the chest over the fullest part of the breasts, subtract the bra size from the size of the chest. If the difference is 0 to 1/2 inch, it is AA cup size, 1/2 to 1 inch, it is A and sizes go up gradually.