How Child BMI-for-Age Percentiles Are Calculated
A child's BMI is calculated exactly like an adult's: weight in kilograms divided by height in meters squared (BMI = kg ÷ m²). But a healthy BMI value changes a great deal as children grow, so pediatricians never compare a child's BMI to the fixed adult cutoffs of 18.5, 25, and 30. Instead, they compare it to the CDC's BMI-for-age growth charts, which show how BMI is distributed among thousands of children of the same age (in months) and sex, and express the result as a percentile.
The LMS method behind the percentile
The CDC growth charts are built from three reference numbers published for each age and sex: L (a skewness/Box-Cox power), M (the median BMI), and S (the coefficient of variation). This calculator interpolates the CDC's published L, M, and S values across 6-month age increments from 2 to 20 years, then converts the child's BMI into a z-score with Z = [(BMI/M)^L − 1] / (L × S), or Z = ln(BMI/M) / S when L is zero. That z-score is converted to a percentile using the standard normal distribution — the same method the CDC's own growth-chart software uses.
Weight status categories
The CDC and American Academy of Pediatrics define four categories from the percentile: underweight is below the 5th percentile, healthy weight spans the 5th up to the 85th percentile, overweight spans the 85th up to the 95th percentile, and obesity is the 95th percentile or above. These bands are set relative to other children of the same age and sex, not to a universal number.
Why kids can't use adult BMI cutoffs
Body fat percentage naturally rises in early childhood, dips around ages 5-6 (the "adiposity rebound"), and rises again through the teen years as puberty progresses — boys and girls also diverge from each other over this period. Applying one fixed adult cutoff at every age would misclassify healthy toddlers as overweight and could miss real weight concerns in teenagers. The percentile approach automatically adjusts for where a child is in that growth curve.