BMI for Children and Teens
Children and teens are never assessed against the fixed adult BMI categories. Instead, their BMI is plotted on an age- and sex-specific percentile chart — the same formula, a completely different interpretation.
Percentile bands
| Percentile range | Category |
|---|---|
| Below 5th percentile | Underweight |
| 5th to 84th percentile | Healthy weight |
| 85th to 94th percentile | Overweight |
| 95th percentile and above | Obesity |
This is a general reference, not a substitute for pediatric growth charts, which plot percentiles as smooth curves across specific ages rather than fixed bands.
Why percentiles instead of fixed categories
A healthy BMI for a 6-year-old is not the same number as a healthy BMI for a 16-year-old — body composition changes predictably throughout childhood and adolescence as part of normal growth. Comparing a child only to other children of the same age and sex accounts for that expected change, which fixed adult thresholds cannot do.
Common questions
- Is child BMI calculated differently from adult BMI?
- The formula — weight divided by height squared — is identical. The interpretation is completely different: instead of fixed adult categories, a child's BMI is plotted against a percentile chart specific to their age and sex.
- What does a BMI percentile mean?
- A percentile shows how a child's BMI compares to other children of the same age and sex in a reference population. A BMI at the 60th percentile means the child's BMI is higher than about 60% of children that age and sex, and lower than about 40%.
- Why can't children use the adult BMI categories?
- Children's healthy body composition changes substantially and predictably as they grow — the same absolute BMI value means something very different at age 6 than at age 16. Fixed adult thresholds would misclassify growing children constantly, so age- and sex-specific percentiles are used instead.
- What should a parent do with a child's BMI percentile?
- Use it as one data point to discuss with a pediatrician, not as a standalone verdict. Growth trajectory over time, family history, and overall development all matter more than a single percentile reading, and a pediatrician is best placed to interpret it in context.