BMI Calculator for Children

BMI-for-age in children is calculated using the same formula as adults (weight in kg ÷ height in m²), but the result is then plotted against age-and-sex-specific growth charts to give a centile. Unlike adult BMI, a fixed number does not define overweight or obese — what is healthy changes as children grow.

The NHS uses the UK-WHO growth charts for children aged 2–18. A BMI above the 91st centile is classified as overweight, and above the 98th centile as obese. Below the 2nd centile may indicate underweight. The calculator returns the centile and classification based on these thresholds.

Children's body composition changes significantly during puberty, so centile tracking over time is more informative than a single reading. If you are concerned about your child's weight, the NHS recommends speaking to your GP or health visitor, who can plot measurements on official growth charts and advise on next steps.

BMI for children — different from adults. Children's BMI assessed against age and sex-specific growth charts, NOT fixed adult thresholds (18.5-25). UK uses WHO and UK90 growth charts. BMI percentile system: under 2nd percentile = underweight; 2nd-91st = healthy; 91st-98th = overweight; 98th+ = obese. NHS National Child Measurement Programme (NCMP): all Reception (age 4-5) and Year 6 (age 10-11) children measured annually.

UK childhood obesity statistics 2026. Reception (age 4-5): 9.6% obese, 12.1% overweight (NHS Digital 2024). Year 6 (age 10-11): 22.1% obese, 14.3% overweight — total 36% above healthy weight. Highest rates in deprived areas (29% Year 6 obesity in most deprived decile vs 13% in least). Ethnic differences: Asian, Black African, Mixed children higher risk. Strong correlation with parental obesity. UK pattern: childhood obesity stable since 2017 (down from peaks).

NHS guidance on healthy weight children. Don't put children under 18 on strict diets without medical supervision. Focus on: 60 minutes daily moderate-vigorous activity (Chief Medical Officer recommendation); 5+ portions fruit and veg daily; limit sugar (under 5% of daily calories); reduce screen time; eat together as family; healthy snacks (fruit, yogurt) replace crisps/sweets. Sleep: 9-12 hours age 6-12; 8-10 hours teens. Restrict ultra-processed foods.

Childhood weight programmes UK. Local Tier 2 services: HENRY (under 5s, family-based), MEND (6-13), Beezee Bodies. NHS Tier 3 specialist (more severe): paediatric weight management clinic. National Child Measurement Programme: results sent to parents — concerning results trigger GP/NHS support offers. School-based: cooking classes, daily mile, healthy lunchtimes. Sugar tax (Soft Drinks Industry Levy 2018+): reduced sugar in soft drinks 30% — saved millions of calories from children's diets.

When to seek medical help. See GP if child consistently above 91st percentile (overweight) or 98th (obese). NOT a casual concern — childhood obesity increases lifetime risk of type 2 diabetes by 5-7×, heart disease 2-3×, certain cancers. Early intervention (under 12) far more effective than later. Eating disorders ALSO require GP referral: sudden weight loss, restrictive eating, vomiting after meals, excessive exercise. Beat eating disorders charity (beateatingdisorders.org.uk) offers UK helpline. Mental health link: weight issues frequently co-occur with anxiety/depression.