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Getting Started: What is Obesity?

Obesity is defined as excess body fat. Because body fat is difficult to measure directly, obesity is often measured by body mass index (BMI), a common scientific way to screen for whether a person is underweight, normal weight, overweight, or obese.

Healthy Weight and Body Mass Index (BMI)

BMI measures weight in relation to height and while it is not a perfect indicator of obesity, it is a valuable tool for public health. Adults with a BMI between 18.5 and 24.9 are normal. Adults with a BMI between 25.0 and 29.9 are considered overweight, those with a BMI of 30 or more are considered obese, and those with a BMI of 40 or more are considered extremely obese.

For children and adolescents, these BMI categories are further divided by sex and age, due to the physical changes that occur during growth and development.

Growth charts from the Centers for Disease Control and Prevention (CDC) are used to calculate children’s BMI. Children and adolescents with a BMI between the 5th and 85th percentile are considered to be a healthy weight. Children in with a BMI between the 85th and 94th percentiles are generally considered to be overweight, and those with a BMI at or above the sex-and age-specific 95th percentile of the population on this growth chart are typically considered obese.

What is a Healthy Weight?

Determining what is a healthy weight for children is challenging, even with precise measures. BMI is often used as a screening tool- since a BMI in the overweight or obese range often, but not always, indicates that a child is at increased risk for health problems. A clinical assessment and other indicators must also be considered when evaluating a child’s overall health and development.

For children and teens, being overweight is defined differently than it is for adults. Children are still growing, and boys and girls develop at different rates.

The American Academy of Pediatrics, in collaboration with the broader medical community, is educating doctors and nurses across the country about obesity, to ensure that medical professionals regularly monitor children’s BMI, provide counseling for healthier eating; and, for the first-time ever, can write a prescription for parents laying out the simple things they can do to increase healthy eating and active play. You can also learn how to assess your child's weight at home.

Calculate Your Child’s BMI Percentile

A child's BMI percentile shows how his or her BMI compared with other boys or girls of the same age. A child or teen that is between the 85th and 95th percentile on the growth chart is considered overweight. A child or teen who is in the 95th percentile or above is considered obese. For children, BMI testing is used to screen them for being overweight, healthy weight, or underweight. It is not a diagnostic tool. For example, a child may have a high BMI for age and gender, but to determine if excess fat is a problem, a medical professional would need to investigate further and perform additional assessments. Talk to your health care provider to learn more.