Body mass index (BMI)

How is body mass index (BMI) in children interpreted differently from adults?

Although BMI is calculated the same way in children and adults (the individual’s weight in relation to height), the results are interpreted differently.

Adults 20 and older fall into different weight status categories that are each associated with BMI ranges:

• underweight: below 18.5
• normal weight: 18.5–24.9
• overweight: 25–29.9
• obese: 30 or greater

The amount of body fat a person has changes with age and varies by sex.  For this reason, BMI for boys and girls between the ages of 2 and 20 are plotted on a separate chart that accounts for these differences.

A child’s resulting BMI percentile falls into one of the following categories:

• underweight: below the 3rd percentile
• healthy weight: 3rd percentile to less than the 85th percentile
• overweight: 85th percentile to less than 95th percentile
• obese: 95th percentile and above
• severe obesity: 99th  percentile and above

Are there any problems to consider when using BMI to screen for weight problems in children and adolescents?

Yes. While BMI calculations are an easy and convenient way to measure body fat, they have certain limitations:

• BMI doesn’t account for muscle mass. Although it’s uncommon, a child with excess muscle mass might fall into an overweight or obese category.

• Secondly, BMI doesn't provide any information about fat distribution, which correlates with risk for heart disease. A child whose fat is primarily distributed around the abdomen (central adiposity) has a greater risk for heart disease than a child with fat distributed primarily around the hips, thighs and buttocks.

• A waist-to-hip ratio is used to measure body fat distribution. While this measure may provide better indication of a child’s body fat and risk of obesity-related heart disease, it’s difficult to standardize and is not used regularly.

It is important to remember that no single measure of body fat can determine obesity-related disease or an individual’s risk of disease.  Your child’s doctor should take all genetic, environmental and medical factors into account during a health assessment. 

How is body mass index (BMI) in children interpreted differently from adults?

Although BMI is calculated the same way in children and adults (the individual’s weight in relation to height), the results are interpreted differently.

Adults 20 and older fall into different weight status categories that are each associated with BMI ranges:

• underweight: below 18.5
• normal weight: 18.5–24.9
• overweight: 25–29.9
• obese: 30 or greater

The amount of body fat a person has changes with age and varies by sex.  For this reason, BMI for boys and girls between the ages of 2 and 20 are plotted on a separate chart that accounts for these differences.

A child’s resulting BMI percentile falls into one of the following categories:

• underweight: below the 3rd percentile
• healthy weight: 3rd percentile to less than the 85th percentile
• overweight: 85th percentile to less than 95th percentile
• obese: 95th percentile and above
• severe obesity: 99th  percentile and above

Are there any problems to consider when using BMI to screen for weight problems in children and adolescents?

Yes. While BMI calculations are an easy and convenient way to measure body fat, they have certain limitations:

• BMI doesn’t account for muscle mass. Although it’s uncommon, a child with excess muscle mass might fall into an overweight or obese category.

• Secondly, BMI doesn't provide any information about fat distribution, which correlates with risk for heart disease. A child whose fat is primarily distributed around the abdomen (central adiposity) has a greater risk for heart disease than a child with fat distributed primarily around the hips, thighs and buttocks.

• A waist-to-hip ratio is used to measure body fat distribution. While this measure may provide better indication of a child’s body fat and risk of obesity-related heart disease, it’s difficult to standardize and is not used regularly.

It is important to remember that no single measure of body fat can determine obesity-related disease or an individual’s risk of disease.  Your child’s doctor should take all genetic, environmental and medical factors into account during a health assessment.