child BMI and growth percentile
A child's BMI uses the same weight-over-height-squared math as an adult's, but the similarity stops there: the result is never compared against the adult's six fixed bands. Instead, it goes into a reference table and becomes a percentile relative to other children of the same age and sex.
A 16.8 BMI lands in the 53.5th percentile for a boy at exactly 120 months on the CDC table, inside the healthy band of 5 to 85.
The same 32 kg weight from the previous example, on a younger and shorter child, produces a higher BMI (19.5) that lands in the 91.8th percentile, already inside the overweight band (85 to 95).
The same absolute BMI value, 19.5, would be healthy weight under the adult's fixed band, but is overweight on the percentile table for an 8-year-old girl, direct proof of why the two classifications cannot be mixed.
Children and teens are still growing; CDC recommends age-and-sex-specific percentiles.
The percentile compares a child's BMI against other children of the same age in months and sex, not against a generic weight range. A BMI that would look ordinary in isolation can sit above most children of that specific age, which is exactly what the percentile is measuring.
The reference used by the calculator covers 24 to 240.5 months, that is, ages 2 to 20. Outside that age range, the percentile calculation does not apply and BMI needs to be interpreted through a different method.
It is the band above regular obesity, defined by the CDC and the American Academy of Pediatrics as a BMI at or above 120% of the 95th percentile for that age and sex, or an absolute BMI at or above 35 kg/m2, whichever is lower. The second criterion exists because, near age 20, 120% of the 95th percentile can already exceed 35 on its own.
The formula is the same (weight over height squared), but the interpretation is different: adults use six fixed BMI bands, while children use a percentile relative to other children of the same age in months and sex, because a child's body composition changes with growth in a way the formula alone cannot capture.
Medical disclaimer: educational tool for ages 2–19. BMI percentile does not replace a pediatrician, dietitian or longitudinal growth assessment.
Percentile from 85 to below 95. It is a signal to discuss with a health professional.
BMI = kg/m²; percentile calculated with the CDC BMI-for-age LMS table (bmiagerev.csv).
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