A child's BMI on its own means nothing at all, and that is the single most useful thing to know before reading one. The same figure is an ordinary reading at one age and an unusual one at another. A BMI of 16.6 sits near the 78th percentile for a five-year-old and near the 1st for a twenty-year-old...
This is a screening figure, not a diagnosis, and one reading tells you very little. Children grow in bursts and their proportions change constantly — a percentile that moves between visits is usually normal growth rather than a problem. BMI also cannot distinguish muscle from fat, or a child who is simply built solidly. It is a prompt for a conversation with a paediatrician, who can see the whole growth curve and the child in front of them. If anything here worries you, that conversation is the useful next step, and nothing on this page is a reason to change what a child eats.
Months matter here. Between two and twenty the reference shifts every month, so a rounded age can move the percentile by several points.
BMI 16.6 SITS AT THE
65th
percentile for a boy of 8 years 4 months — meaning about 65 in every 100 children of the same age and sex have a lower BMI. The CDC calls this the healthy weight range.
BMI
16.6
PERCENTILE
64.7
Z-SCORE
0.38
The same BMI of 16.6 would be the 81st percentile anywhere between 4 and 6 years, and the below the 1st anywhere between 16 and 20 years. Children are not small adults: BMI falls naturally from age two until about five and a half, then climbs through adolescence. That is why a child's BMI on its own carries no meaning at all, and why the percentile — the comparison with children of the same age and sex — is the only figure worth reading.
Age in months
100
BMI
16.57
Percentile
64.67
Z-score
0.376
95th percentile BMI
20.36
Method
CDC 2000 LMS
What this cannot see. BMI is weight against height squared and nothing else — it cannot tell muscle from fat, and a strongly built or athletic child often reads high while being perfectly healthy. It says nothing about diet, fitness, or how a child is doing. Puberty moves it about considerably and at different times for different children. A single point on a chart is also the least informative thing a growth curve contains: what a paediatrician actually looks at is the shape over years, which is why the readings taken at routine appointments matter more than any one calculation.
THE CHART
Notice that every curve dips before it climbs. That is the adiposity rebound, and it happens to all children at around five and a half. A reading that rises after that point is following the normal shape of growth rather than departing from it, which is one of the commonest reasons a parent worries unnecessarily.
CDC growth charts · live calculation
Enter the child’s sex, and their age in years and months. Months genuinely matter — the reference shifts every month between two and twenty, so a rounded age can move the percentile by several points.
Enter height and weight in whichever units you have. The BMI itself is the easy part; what the page does with it is the comparison against children of the same age and sex.
Read the percentile rather than the BMI. The second chart shows where the same BMI would sit at every other age, which is the clearest demonstration of why the raw number carries no meaning.
Take the reading to a routine appointment rather than acting on it. A paediatrician has the whole growth curve, which is the thing that actually informs anything.
Last updated: August 1, 2026 · CDC 2000 growth charts with the 2022 extended method above the 95th percentile · Reference table generated from the CDC data file, not transcribed · A screening estimate, not medical advice.