Body Mass Index (BMI) is an assessment of body weight relative to one's height. An elevated BMI has increased risk factors for many diseases, such as cancer, hypertension, diabetes mellitus, hypercholesterolemia, and atherosclerosis. Also, a higher BMI may increase the risk for a premature death. A healthy BMI is 18.5 – 24.9 kg/m. A BMI of at least 30 kg/m indicates obesity and an increased health risk. A BMI of over 30.0 kg/m indicates grade II obesity, while 40.0 kg/m  and greater is morbid obesity. 

Body mass index (BMI) can easily be used to help establish a client's fitness program, because it is simple to calculate and the results are generally reliable. The BMI may not apply to some individuals with more than normal muscle mass, such as large athletes or bodybuilders. 

BMI is the preferred body composition assessment for the obese population. Fat calipers lose their accuracy with large skin folds and variance in fat density. Bio-impedance and near-infrared typically underestimate body fat percentage in this population. Although BMIs don’t account for body fat percentage, it is not necessary because excess body fat is already known in the obese.

BMI correlates with body fat. The relation between body fat and BMI differs with age and gender. Women are more likely to have a higher percentage of body fat than men with the same BMI. Older adults may have more body fat than younger adults with the same BMI. For personal trainers, it is important to remember that athletes may register a high BMI because of increased muscularity, not increased body fat.

BMI measures your weight in relation to your height, and is closely associated with measures of body fat. You can calculate your BMI using this formula:

For example, for someone who is 5 feet, 7 inches tall and weighs 220 pounds, the calculation would look like this:

A BMI of 18.5 to 24.9 is considered healthy. A person with a BMI of 25 to 29.9 is considered overweight, and a person with a BMI of 30 or more is considered obese.

You can also find your weight group on the chart below. The chart applies to all adults. The higher weights in the healthy range apply to people with more muscle and bone, such as men. Even within the healthy range, weight gain could increase your risk for health problems.

Find your weight on the bottom of the graph. Go straight up from that point until you come to the line that matches your height. Then look to find your weight group. The higher your BMI is over 25, the greater chance you may have of developing health problems.

* Without shoes **Without clothes

Because BMI does not show the difference between fat and muscle, it does not always accurately predict when weight could lead to health problems. For example, someone with a lot of muscle (such as a bodybuilder) may have a BMI in the unhealthy range, but still be healthy and have little risk of developing diabetes or having a heart attack.

BMI also may not accurately reflect body fat in people who are under 5 feet tall and in older people, who tend to lose muscle mass as they age. Keep in mind, BMI may not be the best predictor of weight-related health problems among some racial and ethnic groups. 

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