IS YOUR BODY MASS INDEX (BMI) TOO HIGH, TOO LOW, OR JUST RIGHT?
Body mass index (BMI) is a measure of weight in relation to a person’s height. It is calculated by dividing body weight in kilograms by height in meters squared.
BMI is used to classify individuals as underweight, normal, overweight, or obese and to identify individuals at risk for obesity-related diseases. For most individuals, obesity-related health problems increase with a BMI > 25 kg•m2. A BMI > 30 kg•m2 is associated with hypertension, dyslipidemia, coronary heart disease, and mortality. BMI can be used to classify children and adolescents as overweight or obese using the standard BMI formula along with a BMI for age growth chart provided by the Centers for Disease Control and Prevention. In children and adolescents, overweight is defined as the 85th to <95th percentile of BMI for age and sex, whereas obesity is defined as >95th percentile for age and sex.
The following example can be used to learn how to calculate BMI. An individual weighing 150 lb (or 68.18 kg [divide weight in pounds by 2.2]), standing 66-in tall (or 1.68 m tall [multiply height in inches by 0.0254]) has a BMI of:
BMI = weight (kg)/height (m2)
= 68.18 kg/(1.68 m)2
= 68.18/2.82
= 24.2 kg•m-2
Another formula eliminates the need to convert pounds and inches to kilograms and meters. Using the sample:
BMI = (weight [lb]/height [in2]) x 704.5
= (150 lb/66 in2) x 704.5
= (0.0344352) x 704.5
= 24.2 kg•m-2
BMI does not differentiate between fat and fat-free mass, so it is not a true measure body fatness. Limitations of using BMI to classify individuals as normal, overweight, or obese must be recognized. Individuals with high levels of muscle mass can be misclassified as overweight or, in extreme cases, obese (e.g., body builder). BMI can underestimate body fat and persons who have lost muscle mass (e.g., older adults). High BMI in very short individuals (under 5 feet) may not reflect fatness. Despite these limitations, BMI is a very useful measurement.