BMI (Body Mass Index) for children cannot be interpreted the same way as for adults. Because children grow and their body composition changes dramatically from age 2 to 18, their BMI must be compared to UK reference centile charts, taking into account their age and sex. A BMI that would indicate a healthy weight for an adult might be considered overweight — or underweight — for a specific child.
Adult BMI uses fixed cut-offs: under 18.5 (underweight), 18.5–24.9 (healthy), 25–29.9 (overweight), 30+ (obese). These thresholds apply regardless of age or sex because adult body composition is relatively stable. For children, this approach fails because:
The UK uses the UK90 growth reference charts (Cole et al., 1995, updated), which established centile values for BMI across ages 2–18 separately for boys and girls. The National Institute for Health and Care Excellence (NICE) and the Royal College of Paediatrics and Child Health (RCPCH) endorse these charts.
The NCMP, coordinated by NHS England and local authorities, measures children in:
Results from the 2023–24 NCMP show that around 22.7% of Year 6 children were living with obesity and a further 14.3% were overweight — concerning trends that inform NHS and government public health strategy. Parents receive their child's result by letter.
| Age | Boys — 2nd Centile | Boys — 50th Centile | Boys — 91st Centile | Girls — 50th Centile |
|---|---|---|---|---|
| 2 | 14.5 | 16.4 | 18.3 | 16.2 |
| 4 | 13.9 | 15.6 | 17.6 | 15.4 |
| 6 | 13.5 | 15.3 | 17.5 | 15.2 |
| 8 | 13.7 | 16.0 | 19.0 | 16.0 |
| 10 | 14.2 | 16.9 | 20.4 | 17.0 |
| 12 | 14.9 | 18.0 | 22.0 | 18.3 |
| 14 | 16.0 | 19.4 | 23.5 | 19.5 |
| 16 | 17.2 | 20.8 | 24.8 | 20.5 |
| 18 | 18.5 | 22.0 | 26.0 | 21.3 |
Values are approximate median references for the UK90 chart. Use the calculator above for a personalised centile estimate.
The most important first step is to speak to your GP or health visitor. Avoid focusing on weight or dieting with the child directly, as this can negatively affect self-esteem and risk disordered eating behaviours. Instead:
See a GP to rule out underlying medical causes such as coeliac disease, thyroid conditions, or eating disorders. A dietitian referral may be appropriate to ensure nutritional needs are met during critical growth periods.
BMI is a useful population screening tool, but individual limitations include:
The NHS Eatwell Guide recommends that children's diets include:
Physical activity recommendations: 3–4 hours per day of physical play for under-5s; at least 60 minutes of moderate to vigorous activity daily for children aged 5–18, including muscle-strengthening activities at least 3 days per week.
Children's BMI is interpreted using age- and sex-adjusted centile charts rather than fixed cut-offs, because body fatness naturally changes as children develop, and differs between boys and girls. A BMI that would be considered overweight in an adult might be perfectly healthy for a child of a particular age and sex. The UK uses the UK90 reference charts endorsed by NICE and the RCPCH.
In the UK, a healthy weight for children is defined as a BMI between the 2nd and 91st centile. Below the 2nd centile indicates underweight; between the 91st and 98th centile indicates overweight; and above the 98th centile indicates obese. These thresholds are from the UK90 growth charts and are endorsed by NHS England and NICE.
The NCMP measures the height and weight of children in Reception (ages 4–5) and Year 6 (ages 10–11) in state-maintained schools in England. Data is used to monitor childhood obesity trends and inform public health strategy. Parents receive a letter with their child's BMI result and weight category. Participation is free and the measurements are taken by trained school nurses or healthcare professionals.
Speak to your GP or health visitor for a full assessment. Avoid framing it as a weight problem to the child directly. Focus on healthy family habits: more physical activity, balanced meals, reduced screen time. If BMI is above the 91st centile, a referral to a community weight management programme or dietitian may be offered. If below the 2nd centile, investigations to rule out medical causes are important.
BMI is a useful population-level screening tool but has important limitations for individual assessment. It does not measure body fat directly, can misclassify muscular or very tall children, and does not indicate fat distribution. It should always be considered alongside growth trajectory, clinical assessment, diet, activity levels, and other health factors rather than in isolation.
See a GP if your child's BMI is consistently above the 91st or below the 2nd centile; if they are losing weight unexpectedly or not gaining weight as expected; if their growth has slowed or they are crossing centile lines downward; if you notice signs of disordered eating or body image concerns; or if you simply want professional reassurance. Earlier intervention is more effective than waiting.
No. Adult BMI calculators use fixed thresholds (18.5–24.9 for healthy weight) that are not appropriate for children. Using an adult BMI calculator for a child will produce a meaningless result because children's healthy BMI range changes with age and is different for boys and girls. Always use a children's BMI centile calculator or consult UK90 reference charts.