You'll need a tape measure for this as you can't rely on your clothes size to give a reliable figure. Lift your top and lower your waistband to reveal the bare flesh around your belly button. Your waist will be at, or just above the level of your belly button. Feel for the top of your hip bones and the bottom of your rib cage - your waist is the fleshy part in between and is normally the narrowest part of your torso. Wrap the tape measure around your waist, making sure that it is straight all the way around, and exhale. Read the measurement and enter into your vital statistics.
Waist size is an important measure for cardiovascular risk as well as risk for diabetes. Men should aim for a waist size of less than 94cms (37 inches) and women of less than 80 cms 31.5 inches).