Is Your BMI Measuring Up

Body mass index

The body mass index (BMI), or Quetelet index, is a measure for human body shape based on an individual’s weight and height. It was devised between 1830 and 1850 during the course of developing “social physics”. Body mass index is defined as the individual’s body mass divided by the square of their height. BMI provided a simple numeric measure of a person’s thickness or thinness, allowing health professionals to discuss overweight and underweight problems more objectively with their patients. However, BMI has become controversial because many people, including physicians, have come to rely on its apparent numerical authority for medical diagnosis, but that was never the BMI’s purpose; it is meant to be used as a simple means of classifying sedentary (physically inactive) individuals, or rather, populations, with an average body composition.   For these individuals, the current value settings are as follows: a BMI of 18.5 to 25 may indicate optimal weight; a BMI lower than 18.5 suggests the person is underweight while a number above 25 may indicate the person is overweight; a person may have a BMI below 18.5 due to disease; a number above 30 suggests the person is obese (over 40, morbidly obese).


A frequent use of the BMI is to assess how much an individual’s body weight departs from what is normal or desirable for a person of his or her height. The weight excess or deficiency may, in part, be accounted for by body fat (adipose tissue) although other factors such as muscularity also affect BMI significantly (see discussion below and overweight). The WHO regards a BMI of less than 18.5 as underweight and may indicate malnutrition, an eating disorder, or other health problems, while a BMI greater than 25 is considered overweight and above 30 is considered obese. These ranges of BMI values are valid only as statistical categories

Category BMI range – kg/m2 BMI Prime
Very severely underweight less than 15 less than 0.60
Severely underweight from 15.0 to 16.0 from 0.60 to 0.64
Underweight from 16.0 to 18.5 from 0.64 to 0.74
Normal (healthy weight) from 18.5 to 25 from 0.74 to 1.0
Overweight from 25 to 30 from 1.0 to 1.2
Obese Class I (Moderately obese) from 30 to 35 from 1.2 to 1.4
Obese Class II (Severely obese) from 35 to 40 from 1.4 to 1.6
Obese Class III (Very severely obese) over 40 over 1.6

United States

In 1998, the U.S. National Institutes of Health and the Centers for Disease Control and Prevention brought U.S. definitions into line with World Health Organization guidelines, lowering the normal/overweight cut-off from BMI 27.8 to BMI 25.  The U.S. National Health and Nutrition Examination Survey of 1994 indicated that 59% of American men and 49% of women had BMIs over 25. Morbid obesity—a BMI of 40 or more—was found in 2% of the men and 4% of the women. The newest survey in 2007 indicates a continuation of the increase in BMI: 63% of Americans are overweight or obese, with 26% now in the obese category (a BMI of 30 or more). There are differing opinions on the threshold for being underweight in females; doctors quote anything from 18.5 to 20 as being the lowest index, the most frequently stated being 19. A BMI nearing 15 is usually used as an indicator for starvation and the health risks involved, with a BMI less than 17.5 being an informal criterion for the diagnosis of anorexia nervosa.

Health consequences of overweight and obesity in adults

The BMI ranges are based on the relationship between body weight and disease and death. Overweight and obese individuals are at increased risk for many diseases and health conditions, including the following:

  • Hypertension
  • Dyslipidemia (for example, high LDL cholesterol, low HDL cholesterol, or high levels of triglycerides)
  • Type 2 diabetes
  • Coronary heart disease
  • Stroke
  • Gallbladder disease
  • Osteoarthritis
  • Sleep apnea and respiratory problems
  • Some cancers (endometrial, breast, and colon).