What Exactly Is the Body Mass Index?

The Body Mass Index ( BMI ) can be a useful tool and has become popular in managing weight and body fat percentage over the last 20 years. Calculating BMI 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 you manage your body weight more scientifically.

Body Mass Index measurements help take the subjective, emotional judgments out of losing weight. While emotion can provide motivation and 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 your height into measurement. 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 6 in (66 inches) tall, who weighs 125 lbs the calculation would look like this:

Height: 66 inches x 2.54 cm/inch = 167.64cm = 1.6764 m
Height squared: 1.7018 m x 1.7018 m = 2.810 m^2
Weight: 125 lbs x 0.454 kg/lb = 56.75 kg

So, BMI = 56.75 kg / 2.810 m^2 = 20.19

Nice number, but what does it 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

You can use this free Body Mass Index Calculator if you want a quick answer without the math conversions.

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 Body Mass Index. 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 higher percentage of body fat than men, on average.

Another useful measurement to consider along with body mass index is waist circumference.

Since, for men and women both (though particularly for men) body fat is stored preferentially around the waist this measurement 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.

Body Mass Index and waist circumference don't tell us the whole story about body fat and how to manage it. But these numbers represent useful and objective measures when considering any weight loss program and chances are, if you are taking these measurements, you are concerned about your health. If your numbers are higher than you would like, and you'd like another opinion on the best course to follow to get your weight under control, drop Chad Tacket a line at Chad@global-fitness.com.