Since the 19th century, the body mass index scale (BMI), has been the most reliable way to calculate a healthy body weight. The calculations categorize people into levels of normal weight, underweight, overweight, or obese, depending on their weight to height proportions.

However, because the BMI cannot distinguish between body fat and lean tissue, this measurement is not the most accurate way to assess your health in relation to your body weight. For example, a professional football player who is 6 feet tall and weighs 230 pounds would be considered ‘obese’ according to the BMI scale. But as an NFL athlete, most of that weight comes from lean tissue, not unhealthy body fat. In this case, the BMI wrongly categorizes the player as ‘unhealthy’ because muscle isn’t distinguished from fat.
The second problem with the BMI is the fact that the measurement cannot tell us where the fat in our bodies is located. Fat around our belly is more dangerous than fat around arms and legs, because belly fat is linked to greater risk for chronic diseases like heart disease, diabetes, and high blood pressure.

Instead of relying on the BMI, it would be more beneficial to rely on waist circumference to assess health risks. By measuring waist circumference, you are actually measuring the amount of body fat around your waist that poses more risk than the fat around your arms or legs.

Your waistline, which should measure no more than 40 inches for men and 35 inches for women, is a more accurate screening tool than the BMI when it comes to determining whether you are at risk for developing obesity-related conditions.
The Huffington Post