First thing's first -- grab a measuring tape, put on a comfortable bra that neither enhances nor constricts your true breast size, and make sure you're shopping for the right size bra. If you don't have measuring tape, a piece of string measured against a ruler will do in a pinch.
Step 1: Measure around your chest and upper back, keeping the tape under your armpits (above your breasts). This number is your band size. Note: If you get an odd number, round up one inch.
Step 2: Measure around the biggest part of your chest, being careful to keep the tape level.
Step 3: Subtract your band size from the bigger measurement in Step 2. Every inch of difference between the two measurements represents a cup size. If your band is a 36 and your chest is a 39, then your bra size is 36C. Less then a 1-inch difference is a AA and a 5-inch difference is a DD.