Child BMI Calculator
Calculate BMI-for-age percentile for children and teens aged 2 to 19.
Uses CDC growth charts to display weight status and healthy weight range by height.
Child BMI (Body Mass Index) is calculated using the same formula as adult BMI but is interpreted completely differently. For children and adolescents ages 2–19, BMI is assessed using age- and sex-specific percentile charts from the CDC, because a healthy weight range changes continuously as children grow.
The BMI formula (identical for adults and children):
BMI = Weight (kg) ÷ Height (m)²
In imperial units:
BMI = [Weight (lbs) ÷ Height (inches)²] × 703
Child BMI interpretation — percentile-based:
| Percentile | Category |
|---|---|
| Below 5th | Underweight |
| 5th–84th | Healthy weight |
| 85th–94th | Overweight |
| 95th and above | Obese |
Why percentiles matter: A BMI of 17.5 is classified as “normal” for a 5-year-old boy but “underweight” for a 15-year-old boy. The raw number alone is meaningless without the age and sex context.
Worked example: A 9-year-old girl, 55 lbs, 48 inches tall:
- BMI = [55 ÷ (48²)] × 703 = [55 ÷ 2,304] × 703 = 0.02387 × 703 = 16.8
- Using CDC charts for 9-year-old girls: BMI 16.8 = approximately the 56th percentile → Healthy weight
BMI limitations for children:
- BMI does not measure body fat percentage directly
- Muscular children may register a higher BMI without excess fat
- BMI is a screening tool, not a diagnostic one — a pediatrician’s evaluation is always required for any concerns
Growth monitoring: The CDC recommends plotting BMI at every well-child visit and tracking the trend over time. A single measurement is less informative than a pattern showing whether a child is tracking consistently along a percentile curve or crossing percentile lines.