Band Size (rib cage measurement):
In a non-padded bra, take a tape measure and wrap tightly around your ribcage (bottom of your bust). Ensure the tape measure rests parallel to the ground.
Cup Size Measurement:
Loosely wrap the tape measure around the fullest part of your bust. Take the rib cage measurement and subtract it from the measurement from the fullest part of your bust.
A difference of 1" = A Cup Size, difference of 2" = B Cup Size and so on.