A body mass index, or BMI, is a numerical measurement that helps determine whether you have too much body fat. You can calculate your BMI by dividing your weight in pounds by the square of your height in inches, then multiplying the result by 703. Below 18.5 means you are underweight, between 18.5 and 24.9 means you are normal, between 25.0 and 29.9 indicates you are overweight and above 30.0 indicates obesity.
Myth: A BMI Always Reveals Body Composition
A BMI will not tell you directly how much fat your body has. It is just a numerical relation between height and body weight, so it doesn't differentiate between fat and muscle. In other words, a very muscular person could have the same BMI as an overweight person, though each type of person has a different health outlook. For an accurate analysis of your body composition, doctors use more complicated methods. For example, a bioelectrical impedance analysis uses a low-level electrical current to measure the amount of water in your body. Because muscle contains more water than fat does, doctors can use this measurement to determine exact body composition.
Myth: A Low BMI Means You Are Fit
Having a low BMI doesn't mean you escape health risks related to physical fitness. Other elements, such as diet, work conditions and genetic factors, might cause health problems. For an accurate physical assessment, consult your doctor, who will interview you to form a detailed understanding of your lifestyle and its corresponding health risks.
Myth: All BMI Measurements Should Be Interpreted Similarly
A BMI calculation is the same for everyone, no matter what age. But the proper interpretation of the number depends on the stage of development. For children and teens, body fat changes with age and differs between girls and boys, according to the CDC. For this reason, BMI tables for children and teens take into account both sex and age. BMI tables for adults do not.
Myth: A BMI Is the Only Good Indicator of Obesity-Related Health Risks
A high BMI is not the only fitness indicator you can use to analyze your physical fitness. For example, the circumference of your waist is a predictor of risk for obesity-related diseases, according to the CDC. High blood pressure and physical inactivity also are risk factors for health conditions related to obesity.
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