Child & Teen BMI Calculator

Body mass index for a child is calculated exactly the same way as for an adult — weight in kilograms divided by height in metres squared — but it is interpreted completely differently. A child is not a small adult. Their body composition changes year by year as they grow, so a BMI that would look high in a six-year-old may be perfectly ordinary in a fifteen-year-old, and vice versa.

Clinicians therefore compare a child's BMI against growth reference charts for their age and sex, and describe the result as a percentile rather than a category. Under the widely used CDC charts, a BMI below the 5th percentile is considered underweight, from the 5th to under the 85th is healthy, from the 85th to under the 95th is overweight, and at or above the 95th is obese. This calculator uses a simplified healthy band per age to give you a quick sense of where your child sits, but a proper percentile requires the full chart and, ideally, a plot of the child's trend over time.

That word "trend" is the important one. A single measurement tells you very little. What paediatricians watch is the direction of travel: a child tracking steadily along the 50th percentile for years is a different picture from one who has just crossed two percentile lines upward. Rapid change, in either direction, is the signal worth acting on.

If your child's result looks high or low, treat it as a prompt rather than a verdict. Children should not be put on restrictive diets without medical supervision, because restricting energy and nutrients during growth can harm development. Talk to a paediatrician or a registered dietitian, who can measure properly, look at the whole picture and suggest changes that support growth rather than fight it. This tool gives general information only and is not medical advice or a diagnosis.

This calculator returns estimates for general information only. It is not medical advice, diagnosis or treatment, and it cannot replace an assessment by a qualified health professional. Individual results vary, and figures such as calorie needs, body-fat percentage and heart-rate zones are approximations based on population averages rather than measurements of you.