To check your bra size, you need only take two measurements which are :
Under bust Measurement
Over bust Measurement
  Measure under your bust :
  Add 4ins.
If the measurement obtained is an even number of inches.
  Add 5ins.
If the measurement obtained is an odd number of inches.
(For example, if your under bust measurement is 31 inches add 5 inches. to obtain bra size 36, or, if your bust measurement is 30 inches add 4 inches to obtain bra size 34 )
Measure around the fullest part of your bust to determine cup size.
If the bust measurement is : same as your bra size, your cup size is A.
  1in. more gives Cup size B
2in. more gives Cup size C
(For example, Under bust measurement of 31in. + 5in. = bra size36
Over bust measurement of 38in. = Cup C
  So the correct fitting, would be 36C.