To determine your correct size follow these instructions
It is always better if someone measures you:
A - Bust: Measure over the fullest part of your bust.
B - Waist: Measure around your natural waistline 1-2 inches ABOVE your belly button, which is the SMALLEST point on your waistline; keep the measuring tape snug but not tight.
C - Hips: Measure around the fullest part of your hips, 8" below your waist (Petites, 7")
It is normal for gowns to require some slight alteration in order to fit comfortably.