How to calculate your body mass index

Jun 25, 2010 at 1:42 p.m. ET

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.

Woman on scale

How to calculate your BMI?

The formula for calculating BMI uses weight in kilograms and height in metres:

BMI (kg/m2) = Weight (kg) ÷ Height (m) 2.

It can also be calculated using 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 and obesity

Over 50 healthcare organisations around the world, use the same BMI standards to define adult overweight and obesity.

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.

As BMI increases, so does the risk for several conditions, 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. 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. The abdominal fat carries the greatest health risk.

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