BMI measurements use height and weight to calculate whether someone is overweight. For children, age and gender are also taken into account.
Waist measurement can be used as an additional check, for example in very muscular individuals who may have a high BMI without excess body fat.
Adults who are above a healthy weight have a BMI of 25 or more. If your BMI is in this range but you don't think you are above a healthy weight, you should check with your GP.