The key to determine correct clothing size is taking correct measurements. It is important to know exactly how to measure your body, and to do it carefully.
Bust: Use a flexible measuring tape and measure circumference around the body over the fullest part of the bust, while wearing a correct fitting bra. Make sure the measuring tape is parallel with the floor all the way around.
Waist: Measure circumference around the body at your natural waistline. Your waistline is the narrowest/slimmest part of the torso. Note: Waist measurement is a reference measurement. So make sure to measure at the point where the torso is slimmest, and NOT where i.e. you want your pants to sit (this is a usual mistake). Still not sure where your waistline is? Stand in front of a mirror, and bend to one side. Your waistline is simply where you can see a natural indentation in the torso on this side.
Hips (Seat): Measure circumference of seat at fullest part of your hips.
We use the following size chart for Woman's clothing: