How to Use This Calculator
Enter your child's weight, height, age in months (24+), and sex. The calculator computes BMI and compares it to WHO reference data to determine your child's BMI-for-age percentile and weight classification.
For children under 24 months, the WHO does not use BMI — our weight-for-length calculator is the recommended alternative.
Note: Unlike adult BMI (which uses fixed thresholds), child BMI is age- and sex-specific. A "normal" BMI at age 5 may be "overweight" at age 14.
Understanding BMI-for-Age Percentiles
| Percentile | Classification |
|---|---|
| < 5th | Underweight |
| 5th – 84th | Healthy weight |
| 85th – 94th | Overweight risk |
| ≥ 95th | Obese |
Frequently Asked Questions
What is a healthy BMI for a child?
Unlike adults, a child's healthy BMI depends on their age and sex. WHO uses percentile curves: the 5th to 84th percentile is considered healthy weight. The 85th–94th is overweight risk, and ≥95th is obese.
How is child BMI calculated?
BMI = weight (kg) / height² (m²). For children, this raw value is then compared to WHO age/sex-specific reference populations to determine where they rank among peers. This is the BMI-for-age percentile.
Does my child's BMI change with age?
Yes! BMI changes dramatically during childhood. It drops after infancy (the "adiposity rebound" at ~4-6 years), then rises through adolescence. That's why using BMI-for-age percentiles is essential. A child at the 60th percentile consistently is healthy even as their raw BMI changes.
Track BMI over time
A single percentile is a snapshot. GrowChart tracks the full growth journey.