Body mass index is a comparison of height and weight used to calculate ranges for underweight, average, overweight and obese people across populations. BMI is an easy-to-calculate first step in determining whether you may be at risk for health problems due to your weight. BMI is subject to error from variability in muscle and bone density, especially in children, and is not a diagnosis of being overweight or underweight in itself. It only measures fat percentage.
About BMI
BMI is a simple way to estimate body fat percentage without using a more complicated measurement, such as skin-fold measurement, underwater weighing or X-ray absorption. These methods require trained personnel and specialized equipment. High or low BMI is not a diagnosis of a health problem itself.
Calculating BMI
BMI is calculated from your weight and height. Weigh yourself nude in the morning after a bowel movement but before showering and breakfast to minimize variability. Measure your height by standing barefoot with your back against the wall. If you measured your weight in pounds and height in inches, BMI is your weight in pounds divided by the square of your height in inches. The product is then multiplied by 703. For example, if you weigh 150 lbs. and are 5 feet 5 inches or 65 inches tall, your BMI is equal to 150 divided by the result of 65 times 65, then multiplying by 703 to get 24.96, says CDC.gov.
What To Do With Your BMI
A BMI below 20 is consider underweight, between 20 and 25 is average, between 25 and 30 is overweight and above 30 is obese. If you're outside of the average range of BMI, this suggests that you may be at risk for weight-related health problems. Only your doctor can properly diagnose any specific health problems.
Limitations
BMI overestimates body fat in those with a higher proportion of muscles or high bone density. BMI underestimates body fat in those with weight loss of muscle or bone density, especially elderly people. Due to different growth rates, BMI is not very useful in those younger than 20. Waist girth may also be considered with BMI as a screening tool. Waist girth above 35 inches for women and 40 inches for men raises the risk of heart disease and Type 2 diabetes.
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