Body Mass Index (BMI)

What is It?

Body Mass Index (BMI) is a simple means of placing people into the categories of underweight, normal, overweight, and obese. It has become popular largely because the government and health profession have vigorously promoted it and it requires simple measurements (height and weight) that are generally made during routine physical exams.

How to Determine Your Body Mass Index (BMI)

BMI is calculated as body weight in kilograms divided by the square of height in meters. Using U.S. customary units, it is calculated as 703 times weight in pounds divided by the square of height in inches. For example, if you are 5’10” tall and weight 185 pounds your BMI is 703 x 185/(70 x 70) = 26.5. If you’d rather not calculate your BMI on your own, you can use an online calculator from the U.S. National Heart Lung and Blood Institute to determine your BMI.

Your Body Mass Index (BMI) Weight Category

The following are the categories in which your BMI places you:

  • Underweight: BMI less than 18.5

  • Normal weight: BMI of 18.5-24.9

  • Overweight: BMI of 25-29.9

  • Obese: BMI greater than 30

Health Risks Related to Your Body Mass Index (BMI)

BMI is statistically associated with health risk. For men, a BMI range of about 20-27 has the lowest statistical risk of disease. BMIs below 20 are associated with increased risk of digestive and pulmonary disease, while BMIs above 27 are associated with increased risk of cardiovascular and gallbladder disease as well as diabetes. An Australian study of more than 9,000 people over a 10-year period showed that, as BMI increased beyond 35.0 there was a sharp increase in the rate of cardiovascular disease death. However, a BMI over 35.0 represents extreme (Class II) obesity. A lower degree of obesity does not increase risk of death. A Canadian study of over 11,000 subjects, showed that those with Class I obesity (BMI = 30-34.9) were at normal risk for all-cause mortality. Underweight people (BMI less than 18.5) had a 73% greater risk of dying than those of normal weight (BMI = 18.5-24.9), while people considered overweight (BMI of 25.0-29.9) actually had a 17% reduction in risk of dying. However, it is still desirable to avoid even Class I obesity because diseases associated with it can diminish the quality of life even if modern disease-management via medication can prevent premature death.

Weaknesses of the Body Mass Index (BMI)

There are some major drawbacks to the BMI method. First, it does not measure body fat at all. It merely assumes that, if you fall into a particular bodyweight range for your height, you must be underweight, normal, overweight, or obese. BMI may be somewhat effective for judging whether a given population has become fatter, but it has little accuracy for individuals. There are many people of muscular or stocky build and broad bone structure, but without excess body fat, who fall into the overweight category. For example, National Football League football players average a BMI of about 29, considered overweight, at a body fat of only 12%. NFL defensive linemen average a BMI of about 32, considered obese, at a body fat of only 18%, still within the normal body fat range. Since excess body fat is really the factor that directly affects health, and BMI is only roughly associated with body fat. If your BMI suggests you are overweight, it is best to follow up with one of the more precise methods described in other pages on this site to determine if your body fat is really excessive.

In addition to overestimating the fat in stocky and muscular men, BMI can underestimate the fatness of men of narrow bone structure and low muscularity. Such men may appear lean but carry a significant amount of body fat. Older people who have lost a lot of their muscle mass over the years frequently fall into this category. Even younger people of narrow build and fine bone structure can carry a high percentage of body fat. For example, some Asians tend to be of much narrower bone structure than Caucasians or Blacks and, at a given weight for height, can carry significantly more body fat. As a result, the World Health Organization has noted that, for some but not all Asian populations, a BMI of 22 may be considered overweight in regard to increased health risk, particularly for Type 2 diabetes.

It is important to remember that BMI is a rough screening tool that is much better for characterizing large populations than individuals. Decisions about losing or gaining weight should only be respectively made if either excess fat or emaciation is obvious or more precise measures of body composition are used.

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