Body mass index is a value calculated using a person's weight and height. For most people, BMI is a fairly reliable measure of body fat and is a common screening tool used to identify overweight individuals who could be at increased risk for weight-related health problems. The formula compares the weight of an individual to that of the general population. BMI for children and teens is interpreted somewhat differently than it is for adults.
Calculating BMI
There are two different formulas that can be used to calculate BMI. One formula is to use the metric system dividing weight in kilograms by height in meters squared. Another way to calculate BMI is to divide weight in pounds by height in inches squared and then multiplying by a conversion factor of 703. Accurate height and weight measurements are needed to calculate BMI
Interpreting BMI
There are certain BMI ranges associated with the standard weight status categories. While these categories are the same for women and men of all ages, BMI interpretation for children and teens also relies on age and gender. For adults, a BMI below 18.5 indicates that a person is underweight. Normal weight status has a BMI that ranges between 18.5 and 24.9. BMI values between 25.0 and 29.9 indicate that a person is overweight, while a BMI of 30.0 and above signifies obesity.
BMI for Children and Teens
BMI percentiles are commonly used to assess children's size and growth patterns. The BMI percentiles on a growth chart show a child's position in relation to other children in the U.S. of the same sex and age. However, the amount of body fat for children and teens changes with age. Likewise, there are differences in the amount of body fat between girls and boys.
Caution
According to the Centers for Disease Control and Prevention, although health care providers use BMI to screen for overweight and obesity in children and teens, it cannot be used as an indicator of healthy weight ranges. Healthy weight for children changes as height increases. In addition, healthy weight ranges increase with each month of age for boys and girls.
Reliability as a Predictor
The Centers for Disease Control and Prevention say that although the relationship between BMI and percentage of body fat is generally a dependable predictor of whether a person may develop health problems related to being overweight or obese, there are other factors to consider. Even if the BMI number is the same, women usually have more body fat than men, and older individuals have more body fat than younger adults. Athletes who have a lot of muscle but who have a high BMI might not necessarily be overweight. A high BMI value can be because of a strong build rather than too much body fat.
Health Consequences of High BMI
Too much body fat can put a person at increased risk for heart disease, stroke, hypertension, type-2 diabetes, gall bladder disease, osteoarthritis and some cancers including colon, gall bladder and prostate cancer. Excess fat around the stomach and abdomen put an individual at even greater risk for health problems.
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