Body Mass Index in Children

By The Weight Watchers Research Dept.

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Children are not small adults when it comes to weight and weight management. Use of Body Mass Index (BMI) and weight classifications specifically for children shows how different children are from adults.

BMI-For-Age – Only For Children
Researchers around the world develop their conclusions on children and weight using the same Body Mass Index (BMI) formula that is used for adults. In adults, the same BMI chart applies to everyone. Because children grow rapidly, and boys and girls grow at different rates, children's BMI charts are based on age and gender.

BMI for children is referred to as BMI-for-age because weight classifications are different at every age.¹ A child's BMI, derived from height and weight measurements, can be plotted onto a gender-specific BMI-for-age growth chart and compared to standards for the child's age. Evaluation of BMI-for-age is based on 'percentile', meaning the percentage of children of the same gender and age whose BMI is lower than that of the reference child. For example, for a child at the 60th percentile for BMI-for-age, 60% of children of the same gender and age have a lower BMI.

The BMI-for-age chart helps differentiate between a normal weight gain during growth and excess weight gain. Looks can be deceiving when it comes to weight in children, so healthcare professionals rely on the BMI-for-age chart to guide them. For example, it is not uncommon for infants and toddlers to look 'chubby' but have a BMI in the healthy range.

BMI-for-age is not as simple as weight, but it is a more accurate measure of excess body weight. It corresponds well to levels of body fat and can be used to follow body size from childhood through adolescence and into adulthood.

Children Are Not Obese
Children can be classified into four different categories based on their BMI. Children who are at a 'healthy weight' have a BMI in the healthy range for children of their age. The BMI of 'underweight' children is below the healthy weight range. Two classifications describe children whose BMI is above the healthy weight range – 'at risk of being overweight' and 'overweight'.

'At risk of being overweight' refers to children whose BMI-for-age is between the 85th and 95th percentiles¹. Children in this range are comparable to adults with a BMI of 25 to 29.9, a classification of being overweight. In children, 'overweight' corresponds to a BMI-for-age greater than the 95th percentile¹. This is comparable to adults with a BMI of 30 or more, a classification of obese.

Strictly speaking, there are no obese children. Although the terms 'overweight' and 'obese' often are used when describing a child with excess weight, the term 'obese' does not apply to children.

view footnotes

¹ Centres for Disease Control and Prevention. BMI - Body Mass Index: BMI for Children and Teens.