Child BMI Calculator

The arithmetic is identical to the adult version: BMI = weight ÷ height², weight in kilograms and height in meters. A 10-year-old at 32 kg and 1.35 m gets 32 ÷ (1.35 × 1.35) ≈ 17.6. What changes completely is the interpretation. That 17.6 means nothing on its own until you know how old the child is and whether they are a boy or a girl. The very same figure can sit comfortably in the healthy band for one child and well above it for another two years younger. So the number this page returns is only the first half of the answer. The second half lives in the growth charts, and you need both.

Children are not small adults, and body fat does not hold steady while they grow. It is relatively high in infancy, falls through early childhood, then rises again through the years around puberty — and the pattern differs between boys and girls, and between children maturing at different rates. No fixed cut-off survives that. A threshold that flagged excess fat in a 15-year-old would flag ordinary development in a 6-year-old, and the reverse. The approach used in the United States is BMI-for-age: the child's BMI is compared against reference data for children of the same age and sex, and what you read off is a percentile — the share of that reference group with a lower BMI. The 60th percentile means 60% of the reference children fell below. It describes a position within a distribution; it is not itself a measure of health.

On the CDC charts for ages 2 through 19, the conventional bands are: below the 5th percentile underweight, 5th to below the 85th healthy weight, 85th to below the 95th overweight, and 95th and above obesity. The caution that applies to adult cut-offs applies here with more force. These are lines drawn across a continuous distribution to make population screening possible; nothing meaningful changes about a child between the 84th and the 86th percentile. And BMI in children inherits every blindness of the adult index. It cannot separate muscle from fat, it does not know where fat is stored, and it says nothing about diet, fitness, sleep, puberty stage or growth history.

This calculator deliberately returns the BMI value and stops there. Turning a BMI into a percentile requires the CDC reference tables — age- and sex-specific parameters in monthly steps — and an improvised approximation would produce a figure that looks authoritative while being wrong. The CDC publishes a child and teen BMI calculator that performs the lookup properly, and the clinical growth charts themselves are freely available. Use the value from this page with those, or with a pediatrician who already plots it at every visit. A single reading also matters far less than the trajectory. Children grow in bursts, and a child can cross percentile lines during normal development. One point is a snapshot; a curve across several visits is information.

A result outside the middle bands is a reason to talk to a pediatrician, not a reason to act at home. Growth rate, puberty timing, family history, medication and underlying conditions all shape what a given percentile means, and only a clinician sees the whole picture. That holds in both directions. A percentile that is low, or falling steadily across visits, deserves attention just as much as a high one. It is also worth saying plainly: putting a child on a restrictive diet, or framing a number on a chart as something to feel bad about, carries real risk to eating behaviour and to growth. Any change to what a child eats or how they move belongs in a conversation with their doctor. This page calculates a value and explains how it is read — nothing more.