Why children's BMI is different
Children and teenagers are still growing. A BMI of 19 means something very different at age 6 (high) than at age 18 (low normal). The CDC therefore interprets BMI for ages 2–19 as a percentile against same-age, same-sex peers from the 2000 CDC growth reference cohort. The four bands are: underweight below the 5th percentile, healthy weight 5th to less than 85th, overweight 85th to less than 95th, and obesity 95th and above (severe obesity at 120% of the 95th).
What this calculator does — and does not
This calculator gives you a coarse CDC reference band for a child's BMI. It does not embed the full CDC LMS lookup table, so it cannot return an exact percentile (e.g. "78th percentile"). For an exact percentile you need the LMS parameters at the child's age and sex, or a clinician's growth-chart plot. We show the band so the number has meaning; the BMI charts page has the band definitions and reference links.
Body fat, calories and other panels for children
The Deurenberg body-fat formula was derived on adults, so it is suppressed under age 18. Maintenance calories use Mifflin–St Jeor, which is a reasonable approximation for older teens but is unreliable for younger children. The healthy weight, ideal weight and lean body mass figures are still computed because they help a parent or paediatrician reason about growth trajectory, but for children we recommend treating them as background, not targets.
When to talk to a paediatrician
If your child's BMI falls in the overweight or obese band — or has trended up across several visits — schedule a review. A paediatrician can plot the trend against the WHO/CDC reference, consider family history, and rule out endocrine causes. A single reading in the underweight band is also worth a conversation, especially if it is recent and unexplained.
Next steps
- Teen BMI calculator for ages 13–19
- Kids BMI calculator for ages 2–12
- BMI charts for the CDC reference tables
- Is BMI accurate? — what BMI cannot tell you