Why child BMI needs more context than adult BMI
BMI is calculated the same way for children as adults — weight divided by height squared — but interpreting the result is fundamentally different. A BMI of 18 might be underweight for one age and overweight for another, because what counts as a healthy body composition shifts substantially as children grow and develop.
Proper pediatric practice addresses this using percentile growth charts — comparing a child's BMI against a large reference population of children the same age and sex, rather than against fixed numeric cutoffs. This calculator uses simplified, rough category bands instead, which is meaningfully less precise.
When to see a pediatrician
For any genuine concern about a child's growth or weight, a pediatrician using official growth charts is the appropriate resource — they can track your specific child's percentile over time, which is far more informative than any single BMI snapshot, simplified or otherwise.