1. THE BACK OR BAND SIZE (A)
keep your bra on and measure in inches around your ribcage directly under your bust.
If it is an even number = add 4 ins
If it is an odd number add 5 ins
This gives you the band or back size.
When measureing, you should always round down!
Your underband should be snug, e.g. If you achieve 29.5 ins then round down ot 29 ins.
For example: your underband measurement is 19in then add 5 ins - you are a 34.
2. THE CUP SIZE (B)
Now measure around the fullest part of your bust, the full bust measurement less underband = cup size.
When measuring, you should always round up!
The over bust measurement should always be generous, e.g. If you achieve 34.5 ins then round up to 35 ins.
34 Back size & 34 Full Bust size = A cup
34 Back size & 35 Full Bust size (+1 ins difference) = B cup
34 Back size & 36 Full Bust size (+2 ins difference) = C cup
34 Back size & 36 Full Bust size (+3 ins difference) = D cup... etc.