Child BMI Calculator
Calculate BMI percentile for children and teens aged 2-20 using CDC growth charts.
Returns weight category by age and sex per standard pediatric guidelines.
BMI for children and teens (ages 2–19) uses the same mathematical formula as adult BMI but is interpreted completely differently. Instead of fixed cut-off values, children’s BMI must be compared against age- and sex-specific growth charts using percentile rankings.
BMI formula (same for all ages): BMI = Weight (kg) ÷ Height (m)² BMI (imperial) = 703 × Weight (lb) ÷ Height (in)²
Percentile-based interpretation (CDC/WHO standards):
| Percentile Range | Classification |
|---|---|
| Below 5th | Underweight |
| 5th to below 85th | Healthy weight |
| 85th to below 95th | Overweight |
| 95th or above | Obese |
Why age and sex matter: Children have rapidly changing body composition as they grow. A BMI of 18 is healthy for a 12-year-old girl but underweight for a 17-year-old boy. Fat percentage naturally increases during puberty in girls and decreases in boys — fixed cut-offs cannot capture this variation.
BMI-for-age percentile uses z-scores: z = (BMI − median BMI for age/sex) / standard deviation The z-score is then converted to a percentile using the standard normal distribution.
Worked example: An 8-year-old boy weighs 28 kg and is 130 cm tall: BMI = 28 / (1.30)² = 28 / 1.69 = 16.6 On CDC growth charts for 8-year-old males, BMI of 16.6 falls at approximately the 50th percentile — exactly healthy weight.
The same BMI of 16.6 in a 14-year-old boy would fall at only the 10th percentile — still healthy weight, but leaning toward the lower healthy range.
Clinical note: BMI is a screening tool, not a diagnostic one. Healthcare providers use additional assessments (skinfold thickness, dietary history, activity level) before any clinical classification. Never make dietary changes for children based on BMI alone without medical guidance.