To determine your US dress size, you need to measure your Waist, Hips (Seat) and Bust. But don´t worry, measuring is easy. See illustration and guide below.
Bust
Measure circumference around the bust at the fullest part. Use a flexible measuring tape, and keep it parallel with the floor.
Waist
Measure your body around your natural waistline. Your waistline is usually just above your belly button (narrowest part of torso). Not sure where you waistline is?
Waist
Stand in front of a mirror, and bend to one side. Your waistline is simply where you can see a natural indentation in torso on this side.
Hips (seat)
Measure circumference around your hips (seat) at the fullest part.