Body Mass Index

Judgments about body weight can easily become clouded by one's emotions. While it is certainly good to be passionate about managing your body, you need to get a good grounding in facts, first. BMI is an important tool for achieving at least part of that goal.

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

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 m^2
Weight: 120 lbs x 0.454 kg/lb = 54.48 kg

So, in this example, BMI = 54.48 kg / 2.896 m^2 = 18.81

But what exactly does this number mean? The following table lists one commonly accepted classification, using BMI:

Under 18.5 = Between 18.5 and 24.99 Between 25 and 29.99 Between 30 and 34.99 Between 35 and 39.99 40 and above

Underweight Normal Overweight Obese (Class 1) Obese (Class 2) Extreme Obesity

Of course, anyone who finds themselves near the borderline of one classification shouldn't panic, since these can't be anything but guidelines, and action can be taken to change the situation. Nevertheless, anyone near the higher ranges 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.

As with many such tables, there are limitations on the usefulness of the BMI. It doesn't take into account different body types, athletic conditioning, age, muscle-to-fat ratio and other characteristics. Therefore, it could 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 which is useful to couple with BMI is waist circumference. Since, for men and women both (though particularly for men) body fat is stored preferentially around the waist area, 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.

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