Child BMI Calculator (UK)
BMI in children is interpreted differently from adults. This calculator uses age and sex-adjusted centile charts, as used by NHS and Public Health England.
Child BMI Guidelines
Centiles vs Adult Categories
Adult BMI categories (underweight/normal/overweight/obese) do NOT apply to children. Children's BMI must be compared to other children of the same age and sex. NHS centile bands: under 2nd centile (underweight), 2nd–91st (healthy), 91st–98th (overweight), above 98th (very overweight).
Why Age Matters
Children's body composition changes dramatically with age — they typically lose baby fat, then gain muscle through adolescence. A BMI that would be 'overweight' for a 7-year-old might be perfectly healthy for a 14-year-old with the same number. Always use age-adjusted charts.
When to See a Doctor
This calculator provides an estimate only. A single BMI measurement gives limited information — trends over time and overall health picture matter more. If you're concerned about a child's weight, speak to your GP or health visitor, who will use official UK90 centile charts.
When to Seek Medical Advice
Calculator results are estimates based on population averages and statistical formulas — they are not medical diagnoses. Consult your GP if calculator results suggest a health concern, if you have symptoms requiring attention, or if you are managing a chronic condition where precise monitoring is important. Regular health checks (annual GP review, NHS Health Check for those aged 40-74) provide professional assessment using validated clinical tools. Use health calculators to inform conversations
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