When measuring to determine your size, you should wear an unpadded bra.
Remain in a standing position, keep your back and shoulders straight, and breathe normally.
Step 1: Under bust circumference - Measure around your chest, just under the bust (ensure the tape measure lies straight all the way around and is not too tight).
Step 2: Cup size - Measure around the fullest part of your bust (keep the tape measure slightly loose).