The best way would be to ask her, or sneak into her room and find out.
You may need to find out your girlfriend's ring size without her knowing. You could try asking her friends or family.
You can go to Victoria Secret and they measure you...
the average size is a b cup or a c cup
Kesha's cup size is 34B guys
You can find information online for measuring one's bra cup size on Jockey, Shop 411, Kmart, eHow, Bare Necessities, Wikihow and others. Choosing the wrong bra size can lead to various health problems.
The area around the chest + cup siZe. 34 c. 34 inches aroumd. Cup size C
This will depend on many factors, but if you can measure the band, that would be your number. If you can fit the bra band on someone or something and stuff the cups to be full, you can measure the center of the cup. Subtract the band measurement from the cup measurement. For every inch you add a cup size. 1"=A, 2"=B, 3"=C, etc. Of course this is assuming the bra is new and is not stretched out of shape. If your trying to find out a persons bra size I recommend they get a custom fitting (not a department store fitting) to find the correct size.
One cup of coffee can fit in a coffee cup that is 1 cup in size.
-A cup
Your cup size - is the difference between the measurement around the chest just under the breasts - and the measurement around the fullest part of the breast. If you're not sure - seek advice from a professional... such as a lingerie retailer.
The cup size of a bra is the letter in the size, for example the B in "36B" is the cup size. It represents how much larger the breasts are than the measurement around the rib cage.
Bra sizes have two parts, a band size and a cup size. The band size is the measurement around your rib cage, and the cup size is a letter based on the difference between the band size and the measurement around the largest part of your breasts. Band sizes 30-42 are easy to find in stores, along with cup sizes A-D, and smaller or larger sizes can be found at specialty stores.