Step 1: Taking The Measurements
Two measurements need to be taken. The ribcage circumference (measurement A) and the bust circumference (measurement B). Both measurements need to be recorded in inches. The measuring tape should sit firmly, but not tightly against the skin. You should be breathing out when taking the measurements.
  
Step 2: Determining The Band Size
Measurement A will determine the band size.
28” = size 6                             38" = size 16
30” = size 8                             40" = size 18 
32” = size 10                           42" = size 20
34” = size 12                           44" = size 22
36” = size 14                           46" = size 24
 
Step 3: Determining The Cup Size
Determine the cup size by doing a simple calculation to measure the difference between the ribcage size and the bust size. Measurement B (in inches) – Measurement A (in inches).
Eg Measurement B = 40", Measurement A = 36” = difference of 4” = D cup
Eg Measurement B = 50”, Measurement A = 42” = difference of 8” = FF or G cup
 
Difference of 1 = A cup             Difference of 7 = F cup
Difference of 2 = B cup             Difference of 8 = FF/G cup
Difference of 3 = C cup             Difference of 9 = G/GG cup
Difference of 4 = D cup             Difference of 10 = GG/H cup
Difference of 5 = DD cup          Difference of 11 = H/HH cup
Difference of 6 = E cup             Difference of 12 = HH/J cup