Why adult categories do not apply
BMI is weight divided by height squared, and that formula does not change between a child and an adult. What changes is how to interpret it: in childhood, body fat percentage varies markedly and predictably with age — it drops after the first year, then rises again before puberty in what is called the 'adiposity rebound' — so the same BMI of 17 can be low, normal or high depending on whether the child is 3, 6 or 11.
The solution paediatricians worldwide use is the same as for weight: convert BMI into a percentile within an age- and sex-specific reference chart. This calculator uses the US CDC charts (2000), the most widely cited reference from age 2 onward.
The four official bands
The CDC itself defines four categories from the percentile:
- Below the 5th percentile: underweight.
- 5th to 85th percentile: healthy weight.
- 85th to 95th percentile: overweight.
- Above the 95th percentile: obesity.
A real limit of this method, stated plainly
The CDC itself acknowledges a technical limitation: at the extremes of its charts, the statistical parameter L (which shapes the curve) becomes so negative that the formula stops behaving sensibly for very high BMI values, and can return unreliable percentiles above roughly the 97th. Since 2022 the CDC has published 'extended BMI z-scores' designed for exactly those extreme cases, using a different formula from the standard chart. This calculator uses the standard chart, so for a BMI well above the 97th percentile, the result should be read as a sign that it's worth checking with a paediatrician, not as an exact figure.
What to look at besides the number
BMI does not distinguish fat from muscle: a very muscular child can come out 'overweight' without excess fat. And as with weight, what actually matters clinically is the trend across check-ups — a sharp jump in percentile over a short time says more than a single day's snapshot.