Your body mass index, or BMI, is a number that assesses whether you are at a healthy weight based on estimated body fat, according to the Centers for Disease Control and Prevention. You are less likely to have weight-related health problems if you are in a "normal" BMI range. However, the BMI formula is only a screening tool rather than a way to diagnose weight problems.
Normal Adult BMI
Your BMI number is calculated by dividing your weight in pounds by your height in inches squared, then multiplying that number by 703, according to the Centers for Disease Control and Prevention (CDC). Therefore, if you weigh 140 lbs. and you are 65 inches tall, you would divide 140 by 4,225, then multiply the resulting number by 703 and get a BMI score of about 23.3. Since a "normal" BMI is 18.5 to 24.9, your BMI of 23.3 would be in the "normal" range. However, if your BMI is under 18.5 or at or above 25, you may be in an unhealthy weight category, according to the CDC.
Normal BMI for Kids and Teens
Since kids and teens go through body changes on a regular basis, they can't be measured by the same universal chart as adults, according to Kids Health, a part of the Nemours Foundation. Pediatricians take an extra step and plot kids' and teens' BMI numbers on a chart that compares them to kids of the same sex and age. This helps assess where young people stand based on normal averages at any given time in their development. A "normal" BMI for a child or teen would be a BMI that is at the 5th or under the 85th percentile for all kids of the same age and sex, according to the CDC. A child or teen might have a weight problem if she is either below or above these percentiles.
Considerations
Although the correlation between BMI and body fatness is generally strong, it doesn't always correlate perfectly for all people. For example, women tend to have more body fat than men at the same BMI, and older people tend to have more body fat than younger people at the same BMI, according to the CDC. Additionally, avid athletes may be told that they are "overweight" even if they just carry a lot of muscle weight.
Other Significant Measures
Factors such as your waist circumference, your waist-to-hip ratio and any other risk factors for diseases are as important as BMI in assessing your health. You could be in a "normal" BMI range but still be at risk. For example, women who have a waist circumference of at least 35 inches may be more likely to have health problems than women who have smaller waists, according to the Mayo Clinic. Additionally, people with sedentary lifestyles and those with high blood pressure are more likely to have health problems, according to the CDC.
Further Assessments
Since BMI isn't a diagnostic tool, your doctor will need to perform extra assessments before he can determine whether you have any possible weight problems. If your BMI is above the "normal" range, for example, your doctor might ask you about your family's health history, take your blood pressure, take a skinfold thickness measurement, evaluate your diet and ask you how active you are before he concludes that your BMI is accurate, according to the CDC.
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