Measure under your bust (measurement A)

Then add 4 inches. if the measurement is an even number of inches, or add 5 inches. if the measurement is an odd number of inches.

Example
If your underbust measurement is 30inches add 4inches. to give a bra size 34 or, if your underbust measurement is 31inches. add 5inches. to give a bra size 36.)

Overbust Measurement (measurement B)

Measure round the fullest part of your bust to determine Cup size.
If the bust measurement: is:

The same as your bra size, your Cup size is AA.

1in. more gives Cup size A
2in. more gives Cup size B
3in. more gives Cup size C
4in. more gives Cup size D
5in. more gives Cup size DD
and so on through E, EE, FF and G