The Body Mass Index

What BMI means, how it affects your health, and how to calculate yours.
health and weight

Body mass index, or BMI, is a measurement that evaluates the relationship between body weight and height. While BMI is not a direct measure of excess body fat, it is the recommended method to diagnose overweight and obesity. Because it expresses the weight-height relationship, BMI provides a more accurate measure than body weight alone.

The formula for calculating BMI uses weight in kilograms and height in meters: BMI (kg/m2) = Weight (kg) ÷ Height (m) 2; or weight in pounds and height in inches: BMI (lb/in2) = Weight (lb) ÷ Height (in) 2 x 703. Online calculators that automatically do the computations from an entered height and weight are a convenient method of determining BMI.

Definitions of Overweight, Obesity
Over 50 healthcare organizations around the world, including the National Institutes of Health, use the same BMI standards to define adult overweight and obesity.1

Definition BMI
Overweight 25-29.9
Obese 30-39.9
Morbidly Obese 40+

BMI's Link to Diseases
BMI is used as the standard to diagnose overweight and obesity because there are so many studies that show a link between BMI, the risk of several diseases and death.2

As BMI increases, so does the risk for several conditions3, including

  • Diabetes
  • Cardiovascular disease
  • Stroke
  • Hypertension
  • Gallbladder disease
  • Osteoarthritis
  • Sleep apnea
  • Some cancers
  • Premature death

While the links between BMI and disease risk is clear, it is important to remember that it is only one of several disease risk factors. In other words, BMI cannot tell an individual that he or she will get a disease, only that his or her risk of developing the disease is increased.

BMI has some limitations.4 It tends to overestimate body fat in people who are very muscular and underestimate body fat in people who are highly sedentary. BMI also doesn't show where the body fat is located. Abdominal fat carries the greatest health risk.

view footnotes

The Weight Watchers Approach:

The Weight Watchers Healthy Weight Ranges are based on Body Mass Index. Recommended weight goals for adults are equivalent to a BMI of between 20-25.

This content is reviewed regularly. Last updated December 17, 2011.


Other Science Library Topics:

1Kiernan M, Winkleby MA. Identifying patients for weight-loss treatment: an empirical evaluation of the NHLBI obesity education initiative expert panel treatment recommendations. Arch Intern Med. 2000 Jul 24;160(14):2169-76.

2World Health Organization. Physical status: The use and interpretation of anthropometry. Geneva, Switzerland: World Health Organization 1995. WHO Technical Report Series.

3Calle EE, et al. Body-mass index and mortality in a prospective cohort of U.S. adults. N Engl J Med. 1999 Oct 7;341(15):1097-105.

4Strauss BJ. Nutrition diagnoses. Asia Pac J Clin Nutr. 2004 Aug;13(Suppl):S11.