Child BMI Calculator & Percentile (Ages 2–20)

Adult BMI uses fixed cut-offs (underweight <18.5, healthy 18.5–25, overweight 25–30, obese ≥30) that apply to all adults regardless of age and sex. For children, BMI changes dramatically as they grow and differs by sex — so the CDC uses percentiles based on a reference population of the same age and sex. A BMI of 17 might be healthy for a 5-year-old but underweight for a 15-year-old.

CDC categories: Underweight = below 5th percentile. Healthy weight = 5th–84th percentile. Overweight = 85th–94th percentile. Obese = 95th percentile and above. These categories help identify children who may need guidance around nutrition and activity.

Not necessarily. BMI is a screening tool, not a diagnostic test. Muscular or larger-framed children may have a high BMI percentile without excess body fat. Conversely, a child with a healthy BMI but low muscle mass may still have metabolic risks. Your pediatrician uses BMI alongside diet history, family history, activity level, and physical exam to assess health.

Not necessarily. A BMI of 18 would be checked against the CDC BMI-for-age-and-sex chart. For a 3-year-old boy, a BMI of 18 is well above the 97th percentile and would be flagged for review. For a 15-year-old boy, BMI 18 is normal. Always use age- and sex-specific percentile charts for children.

Most children's BMI decreases from infancy through about age 4–6 (as they get taller but weight grows more slowly relative to height), then rises again — this is called adiposity rebound. Early rebound (before age 4) is associated with higher risk of later overweight. Tracking the BMI trajectory over time is more informative than a single reading.

Restrictive dieting is generally not recommended for children whose BMI is high but who are still growing. Instead, the AAP recommends age-appropriate portion sizes, reduced sugar-sweetened beverages, increased physical activity, and healthy family eating habits. For children with BMI ≥95th percentile, involve your pediatrician or a pediatric dietitian.