BMI Terms & Conditions

Body mass index, or BMI, is a measure of a person's body fat based on their height and weight calculations. It is not a direct measure of body fat, but it is used to gain a reliable assessment of your body fat and determine if you are at risk for other health conditions due to your weight. The Centers for Disease Control and Prevention uses BMI to screen individuals for different weight categories as it is an inexpensive method that is easy to calculate.

Calculation

To calculate your BMI, divide your weight in pounds by your height in inches squared and multiply the result by 703. For example, if you are 5 feet, 9 inches and weigh 150 lbs., you would divide 150 by 69 squared, or 4761. You would then multiply the result by 703, giving you a BMI of 22. The calculation for children and teens is slightly different and also takes gender into account.

Classifications

The CDC lists four general categories for BMI. A BMI below 18.5 is considered underweight. A normal BMI falls between 18.5 and 24.9. Anything above 25 is considered overweight, and above 30 is considered obese. Some medical professionals may draw even greater distinctions in the obese category, classifying individuals with a BMI more than 35 as severely obese, a BMI more than 40 as morbidly obese, and a BMI more than 50 as super obese.

Obesity Comorbidities

Obese individuals are at risk of a number of related health problems, commonly referred to as comorbidities. A comorbidity is the presence of one or more disorders in addition to a primary illness. When comorbidities are present, they can act to worsen other existing medical conditions. Common comorbidities associated with obesity include heart disease, type 2 diabetes, high blood pressure, stroke, certain types of cancer, sleep apnea, polycystic ovary syndrome, fertility problems, certain types of cancer and an increased risk of death from all causes.

Conditions Associated With Low BMI

Individuals with a low BMI are also at risk of certain health conditions, including nutritional deficiencies, a weakened immune system, infertility and osteoporosis. Underweight adults also experience an increased risk of complications after major surgery, according to a 1993 study from Auckland Public Hospital in New Zealand, as well as an increased risk of all-cause mortality.

Considerations

BMI can help screen for health risks, but it is not an accurate measure of a person's health. For example, a person who is very muscular may have a BMI more than 25 but may not have a lot of body fat. Also, a person who falls within a normal BMI may have poor nutrition or other lifestyle factors that put them at risk of disease. As noted by the CDC, weight is only one factor related to a person's overall health and risk of disease. BMI should be viewed in conjunction with a person's diet, exercise frequency and overall fitness level.