What is BMI (body mass index)?

BMI is a standard calculation of body weight in relation to height, and is used to diagnose overweight and obesity.

Find your BMI

The Body Mass Index is a standard calculation of body weight in relation to height, and is used to diagnose overweight and obesity.

It’s a good indicator of the amount of body fat a person carries, which in itself is the root of most weight-related health problems. While it doesn’t directly measure excess body fat, BMI is still the recommended method to diagnose overweight and obesity because factoring in height makes it a more accurate measure than body weight alone. There are many studies from around the world that show a link between BMI and the risk of several diseases, and death.[i]

The reliability of the BMI measurement as an indicator of health is one reason why Weight Watchers bases its Healthy Weight Ranges on it.


What your BMI means:

Under 18.5 –  considered underweight and possibly malnourished.

18.5 to 24.9 – within a healthy weight range for young and middle-aged adults.

25.0 to 29.9 –  considered overweight.

Over 30 – considered obese.

As BMI increases, so does the risk for several conditions[ii], including diabetes, cardiovascular disease, stroke, hypertension, gallbladder disease, osteoarthritis, sleep apnea, some cancers and premature death.


The good news about BMI

The great news is that you don’t need to lose a lot of weight to make real and significant improvements to your health. For most people, losing 5 percent to 10 percent of their initial body weight can have plenty of health benefits. These include lowering your BMI, lowering your body’s cholesterol levels, improving blood sugar control and decreasing your risk of sudden death from heart disease or stroke. It can also lighten your wallet — lowering your BMI can stop the need for regular medication.

The BMI recommendations for a healthy weight are the same for men and women. Some challenge this, thinking that the healthy weight for a man of the same height as a woman would be higher since men tend to "look better" at a higher BMI than women. But experts rely on scientific data of medical outcomes and leave "what looks good" out of the analysis. This is because they’ve learned that, as BMI increases above 25, so does ill health. Moreover, the ill-health effects show up at about the same BMI for men and women.

[i] World Health Organization. Physical status: The use and interpretation of anthropometry. Geneva, Switzerland: World Health Organization 1995. WHO Technical Report Series.
[ii] Calle 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.