Before we go to the chart, here's a short explanation of how the Body Mass Index is measured so you can get a better understanding of what it means...

What are overweight and obesity?

Overweight refers to an excess of body weight compared to set standards. The excess weight may come from muscle, bone, fat, and/or body water. Obesity refers specifically to having an abnormally high proportion of body fat. A person can be overweight without being obese, as in the example of a bodybuilder or other athlete who has a lot of muscle. However, many people who are overweight are also obese.

How are overweight and obesity measured?

A number of methods are used to determine if someone is overweight or obese. Some are based on the relation between height and weight; others are based on measurements of body fat. The most commonly used method today is body mass index (BMI).

BMI can be used to screen for both overweight and obesity in adults. It is the measurement of choice for many obesity researchers and other health professionals, as well as the definition used in most published information on overweight and obesity. BMI is a calculation based on height and weight, and it is not gender-specific. BMI does not directly measure percent of body fat, but it is a more accurate indicator of overweight and obesity than relying on weight alone.

BMI is found by dividing a person’s weight in kilograms by height in meters squared. The mathematical formula is:

weight (kg) / height squared (m²).

To determine BMI using pounds and inches, multiply your weight in pounds by 704.5,* then divide the result by your height in inches, and divide that result by your height in inches a second time. (Or you can use the BMI calculator at http://www.nhlbisupport.com/bmi/, or check the chart shown below that has calculated BMI for you.)

* The multiplier 704.5 is used by the National Institutes of Health. Other organizations may use a slightly different multiplier; for example, the American Dietetic Association suggests multiplying by 700. The variation in outcome (a few tenths) is insignificant.

An expert panel convened by the National Heart, Lung, and Blood Institute (NHLBI) in cooperation with the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), both part of the National Institutes of Health (NIH) identified overweight as a BMI of 25–29.9 kg/m², and obesity as a BMI of 30 kg/m² or greater. However, overweight and obesity are not mutually exclusive, since people who are obese are also overweight.[1] Defining overweight as a BMI of 25 or greater is consistent with the recommendations of the World Health Organization [2] and most other countries.

Calculating BMI is simple, quick, and inexpensive—but it does have limitations. One problem with using BMI as a measurement tool is that very muscular people may fall into the "overweight" category when they are actually healthy and fit. Another problem with using BMI is that people who have lost muscle mass, such as the elderly, may be in the "healthy weight" category—according to their BMI—when they actually have reduced nutritional reserves. BMI, therefore, is useful as a general guideline to monitor trends in the population, but by itself is not diagnostic of an individual patient’s health status. Further evaluation of a patient should be performed to determine his or her weight status and associated health risks.

* Source: United States National Institute of Diabetes & Digestive & Kidney Diseases of the National Institutes of Health - NIDDK Weight-Control Information Network

Click here for a printer friendly version of the chart and full article from NIDDK Statistics Related To Overweight and Obesity (opens in a new browser window).