What Is the Body Mass Index?

The BMI (Body Mass Index) has become a useful tool in managing weight and body fat percentage in the last 20 years. Calculating it requires only simple arithmetic and can be performed by anyone. It's important because it provides an objective measurement that, combined with the appropriate scale for age and body type, helps someone manage their body weight more scientifically.

Judgments about body weight can easily become clouded by emotionalism. It's good to be passionate about managing your body, but you need to get a good grounding in facts, first. BMI is an important tool for achieving that goal.

BMI factors in not only your weight, but also your height. Simply divide your weight in kilograms (1 lb = 0.454 kg) by the square of your height in meters. (1 inch = 2.54 cm)

So, for a person 5 ft 7 in (67 inches) tall, who weighs 120 lbs the calculation would look like this:

Height: 67 inches x 2.54 cm/inch = 170.18 cm = 1.7018 m
Height squared: 1.7018 m x 1.7018 m = 2.896 m²
Weight: 120 lbs x 0.454 kg/lb = 54.48 kg

So, BMI = 54.48 kg / 2.896 m²  = 18.81

But what does this number mean?

The following table lists one commonly accepted classification, using BMI:

Under 18.5 = Underweight Between 18.5 and 24.99 = Normal Between 25 and 29.99 = Overweight Between 30 and 34.99 = Obese (Class 1) Between 35 and 39.99 = Obese (Class 2) 40 and above = Extreme Obesity

Of course, anyone near the borderline of one classification shouldn't panic, since these can't be anything but guidelines. Nevertheless, anyone nearer the higher range should consider the health risks associated with a high BMI. Some of those are: hypertension, increased risk of cardiovascular disease (heart problems) and increased chance of diabetes. Consult a physician for details.

There are limitations on the usefulness of BMI. It doesn't take into account different body types, athletic conditioning, age, muscle-to-fat ratio and other characteristics. As a result, it can overestimate the risk for stocky athletes and underestimate it for older individuals who have reduced muscle mass. And, gender isn't taken into account either. Yet women, just as one example, have a naturally larger percentage of body fat than men, on average.

Another measure is useful to couple with BMI: waist circumference. Since, for men and women both (though particularly for men) body fat is stored preferentially around the waist this can be a useful piece of information. For most men around, say, 5 ft 9 inches a waist measurement over 37 inches (94 cm) is substantial, while one over 40 inches (101.6 cm) indicates a health risk. For women approximately 5 ft 7 inches tall, the numbers are 31 inches (78.7 cm) and 35 inches (89 cm), respectively.

Keep in mind that these are averages, but those with substantial waistlines can see the amount of excess fat stored, confirming that the numbers constitute a useful piece of information.

What to do with, or about, those numbers is a different story, of course. No single measurement tells the whole story about weight, body fat and how to manage it. But these represent useful and objective measures when considering any weight loss program.

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