Childs BMI Calculator Child & Teen BMI Calculator Calculate BMI and percentile rankings for children and teens (ages 2 through 19). Child’s Sex Boy Girl Age (Years) Height (Feet & Inches) Weight (lbs) Calculate BMI Reset Calculated BMI 0.0 Healthy Weight Why Percentiles Matter: For children, BMI is interpreted using age-and-sex-specific percentiles because body composition varies as they grow. Healthy weight: 5th to 85th percentile. Overweight: 85th to 95th percentile. Obese: 95th percentile or greater.