You need two measurements and these should be taken when you are wearing your most comfortable bra.
Firstly, measure your bra (back) size
To measure your bra size take the measurement around your body directly under your bust. If the measurement is an even number, add 4 inches (10cm). If the measurement is an odd number add 5 inches (12.5cm) - e.g. if the measurement is 34 inches then add 4 inches = 38 inches. Your correct bra size is therefore 38 inches.
Secondly, measure your cup size
To measure your cup size - take the measurement of the fullest part of your bust. The difference between this measurement and your bra size is your cup size. - e.g. if your bra size is 34 inches and the measurement of the fullest part of your bust is 35 inches, the difference between the two is 1. Based on the information below your cup size is A.
If the difference between your back size and cup size is
1 inch - your cup size is A
2 inches - your cup size is B
3 inches - your cup size is C
4 inches - your cup size is D
5 inches - your cup size is DD
6 inches - your cup size is E
7 inches - your cup size is F
8 inches - your cup size is FF
9 inches - your cup size is G
10 inches - your cup size is GG
11 inches - your cup size is H
12 inches - your cup size is HH
13 inches - your cup size is J
14 inches - your cup size is JJ
Please remember that this is only a guide and that with different bra manufacturers, sizes may vary. You should try on the bra and check that the wires are flat against your skin and that your breasts are fully enclosed without any bulges. 
