STEP 1:
Measure around your body, just under your breast (Band Size)

STEP 2:
Measure around your body, at the fullest part of the breast (Bust Size)

STEP 3:
The difference between the Band Size and the Bust Size shows your Cup Size:
 
10 - 12 cm = Cup A
12 - 14 cm = Cup B
14 - 16 cm = Cup C
16 - 18 cm = Cup D
18 - 20 cm = Cup DD
20 - 22 cm = Cup F
22 - 24 cm = Cup G
24 - 26 cm = Cup H
26 - 28 cm = Cup I
28 - 30 cm = Cup J

STEP 4:
To find your bra size, use the Band Size and the Cup Size.
 
Example:
If your Band Size is 80 cm und your Cup Size is C, you need a size S (small).