Body mass index, or BMI, is a health-screening tool designed to categorize a person’s weight. Although BMI is not a direct way to measure fat, it takes both your weight and your height into account and so is considered a more accurate assessment of fat than body weight alone. While your physician may use your BMI score to categorize you as underweight, normal weight, overweight or obese, the Centers for Disease Control and Prevention uses national BMI statistics to keep better track of weight trends across the country. Median BMI, which refers to the middle value of a given population’s BMI statistics, is used to define the central tendencies for BMI levels in the United States.
In the United States, BMI is calculated by dividing body weight in pounds by height in inches squared. Because the original BMI equation uses metric measurements, you must multiply this result by a conversion factor of 703 to reach the final BMI value:
weight in pounds / (height in inches x height in inches) x 703 = BMI
Using this calculation, someone who is 65 inches tall and weighs 140 pounds would have a BMI of 23.29:
140 / (65 x 65) x 703 = 23.29
The four main BMI categories are defined by specific parameters. A BMI that falls below 18.5 is considered underweight, while a BMI that falls between 18.5 and 24.9 is considered healthy, or normal. A BMI between 25 and 29.9 is classified as overweight; obesity is defined by a BMI score of 30 or higher. Although BMI isn’t always accurate and is not considered a diagnostic tool, it has been found to correlate with certain health risks. Specifically, the risk of cardiovascular disease, type-2 diabetes and mortality increases as BMI levels increase. According to the Harvard T.H. Chan School of Public Health, these risks increase proportionately as a person's BMI goes above 21.
Median American BMI
The National Health and Nutrition Examination Survey, which is released every few years by the U.S. Department of Health and Human Services, contains extensive, representative data on body measurements and BMI scores for Americans of all ages, both genders and various ethnicities. The most recent release, which includes data collected from 2009 to 2010, provides several national BMI statistics, including median BMI levels. According to the survey, the median BMIs of men and women over the age of 20 are 27.8 and 27.3, respectively. Both of these scores qualify as overweight, and both represent an increase in the median BMI through the previous decade: in 2000, both men and women had a median BMI of 26.8.
It’s important to note that median BMI is not the same as mean BMI -- median BMI reflects the midpoint, with half of the people above and half below that value, while mean BMI reflects the combined average of all BMIs within a population, which can easily be skewed by data clusters on either end of the spectrum. Because obesity rates are high in the United States, this same survey lists the average, age-adjusted BMI for American men and women as 28.7, about a full point higher than the median BMIs for both genders.
Ideal Median BMI
The median BMI of an adult population should, ideally, fall somewhere in the range of 21 to 23, according to the World Health Organization. This range helps achieve optimum health within any given adult population. The WHO also notes that individuals can promote good health and decrease their risk of the chronic problems and illnesses associated with higher BMI scores -- including high blood pressure, high cholesterol, high triglycerides, insulin resistance, heart disease, type-2 diabetes and risk of stroke -- simply by striving to maintain a normal BMI, or anywhere in the range of 18.5 to 24.9.
To nudge your BMI into a healthier range, it’s best to take a dual approach: Overhaul your diet and get moving. A high-fiber diet that’s rich in vegetables, fruit, whole grains and lean proteins, and low in processed foods, added sugars and saturated fat, can go a long way in promoting healthy body weight, particularly when coupled with exercise. Simply taking a 20-minute walk two or three times a day can be enough to jump-start weight loss.
Usefulness and Reliability
BMI is a useful tool to help achieve or maintain a healthy BMI, or weight range, and can be more useful than aiming to reach or maintain a specific number on the bathroom scale. BMI statistics, including median BMI figures, also give an accurate picture of a population’s overall weight trends and reveal just how much weight Americans have gained from the early 1960s to 2010. According to the National Institute of Diabetes and Digestive and Kidney Diseases, about 13 percent of American adults qualified as obese in 1962; that number climbed to 36 percent by 2010.
BMI isn’t without its limitations, however. Because it doesn’t distinguish between lean tissue and fatty tissue, it’s not always a reliable indicator of body fatness. For example, it often defines muscular athletes as overweight or obese because of the density of muscle tissue. Assessing a person’s body composition directly by measuring body fat percentage is better done through hydrostatic weighing or the skinfold test. BMI also doesn’t account for fat distribution. Someone who has a normal BMI but carries too much excess belly fat may still be at a higher risk of weight-related health problems because abdominal fat is associated with the same risks as a high BMI. In this case, measuring waist circumference may be a better predictor of health risk for some people.