HOW TO SIZE FOR BRA
Step One: Band Measurement
Using the measuring tape, measure the circumference around your rib cage, pulling the measuring tape snug under your arms and just under your breasts.
Add three inches to this measurement.
If this number is an odd number, round up to the next even number. This is your band size.
Step Two: Cup Measurement
Pull the measuring tape secure across your back and measure around the fullest part of your breasts, making sure you do not constrict your breasts with the measuring tape.
Subtract the band measurement from from cup measurement to determine your cup size.
Cup Size Difference Between Band and Cup Measurement
(in inches)
A 
0 to 1.5
0 to 1.5 B 1.5 to 2.5
1.5 to 2.5 C 2.5 to 3.5
2.5 to 3.5 D 3.5 to 4.5
3.5 to 4.5 E(DD) 5 to 6
F(DDD) 6 to 7
G 7 to 8
7 to 8 H 8 to 9
8 to 9 I 9 to 10
9 to 10 J 10 to 11
10 to 11 For Example...
If your band measurement was 40 inches, you'd add 3 inches to get 43 and then round that number up to the nearest even number, 44.
Then, if your cup measurement was 48, you'd subtract the band measurement from the cup measurement and get 4. Using the chart above, your cup size would be D, and you should be shopping for a 44 D nursing bra.
