For children and teens (2-18 years), BMI is calculated the same way as adults but interpreted using age and sex-specific centile charts.
Why Child BMI is Different
BMI Centile Categories
| Centile | Category | Meaning |
|---|---|---|
| Below 2nd | Underweight | Lower than 98% of children |
| 2nd-90th | Healthy weight | Similar to most children |
| 91st-97th | Overweight | Higher than 91-97% of children |
| 98th and above | Very overweight | Higher than 98% of children |
Try Our Free BMI Calculator for Kids
Check your child's BMI using age and gender-appropriate calculations. Get instant results with our BMI Calculator for Kids. You may also find our BMI Calculator, Baby Weight Calculator and Ideal Weight Calculator useful.
How to Calculate
Example: 10-Year-Old Boy
Height: 140 cm
Weight: 32 kg
BMI: 32 ÷ (1.4 × 1.4) = 16.3
Then plot on growth chart for 10-year-old boys
Result: 50th centile = healthy weight
National Child Measurement Programme
The NHS measures children in Reception (age 4-5) and Year 6 (age 10-11):
- Height and weight are measured at school
- Results sent to parents/carers
- Helps track population health trends
- Parents can opt out
Healthy Habits for Children
- Physical activity: 60+ minutes daily
- Screen time: Limit recreational screens
- 5-a-day: Fruits and vegetables
- Portion sizes: Appropriate for age
- Regular meals: Including breakfast
- Sleep: 9-12 hours depending on age
When to Seek Help
Speak to your GP or health visitor if:
- Your child is above the 98th or below the 2nd centile
- There's a sudden change in weight pattern
- Your child is concerned about their weight
- You need advice on healthy eating for children
How Child BMI Centiles Are Calculated
Unlike adult BMI, which uses fixed thresholds (18.5-24.9 for healthy weight), child BMI must account for the dramatic changes in body composition that occur during growth. The methodology relies on large reference populations to create age- and sex-specific distribution curves known as centile charts.
The BMI Calculation
The raw BMI formula is identical to the adult version: weight in kilograms divided by height in metres squared. However, a BMI of 18 might be perfectly healthy for a 12-year-old but underweight for a 16-year-old. This is why the raw number alone is meaningless without the centile context.
Z-Scores and Clinical Use
Clinicians often convert centile positions into z-scores (standard deviation scores) for more precise tracking. A z-score of 0 represents the 50th centile (median). A z-score of +1.33 corresponds roughly to the 91st centile (overweight threshold), while +2.0 corresponds to the 98th centile (very overweight). Z-scores are particularly useful for tracking changes over time, as a shift of 0.67 in z-score is considered clinically significant and warrants further investigation.
Growth Velocity Considerations
Single BMI measurements provide a snapshot, but healthcare professionals look at the trajectory over time. A child who has consistently tracked on the 80th centile since infancy is in a very different situation from one who has rapidly crossed from the 50th to the 80th centile over six months. Growth velocity is therefore as important as the absolute centile position. The RCPCH (Royal College of Paediatrics and Child Health) recommends plotting at least three measurements over time before making clinical decisions.
UK-Specific Context: Child Weight in Britain
Childhood obesity is one of the most significant public health challenges facing the UK. Understanding the national picture helps parents and carers contextualise their child's measurements within the broader population.
The National Child Measurement Programme (NCMP)
England's NCMP is one of the largest child surveillance programmes in the world, measuring approximately one million children annually. Children are weighed and measured during Reception year (ages 4-5) and Year 6 (ages 10-11). The programme, established in 2006, provides crucial data on population-level trends and helps local authorities target resources where they are most needed.
Scotland operates a similar programme through the Child Health Surveillance Programme, while Wales runs the Child Measurement Programme for Wales. Northern Ireland conducts measurements through the Health and Social Care Board. Each devolved nation publishes its own statistics, though the broad trends are similar across the UK.
Deprivation and Obesity
NCMP data consistently shows a stark relationship between deprivation and childhood obesity. Children in the most deprived areas of England are more than twice as likely to be obese as those in the least deprived areas. By Year 6, the gap is even wider: approximately 27% of children in the most deprived quintile are classified as obese, compared to around 12% in the least deprived. This inequality has widened since the COVID-19 pandemic, with lockdowns disproportionately affecting physical activity and diet quality in disadvantaged households.
UK Government Programmes
Several government-funded programmes aim to address childhood obesity. The NHS Healthier Families campaign provides free recipes, activity ideas, and food scanner apps. The School Fruit and Vegetable Scheme gives children aged 4-6 a free piece of fruit or vegetable each school day. The Soft Drinks Industry Levy (sugar tax), introduced in 2018, has led to significant reformulation of sugary drinks. Local authorities also commission tier 2 and tier 3 weight management services through the NHS, providing specialist support for children above the 98th centile.
Worked Examples
Example 1: Reception-Age Girl
Age: 4 years 8 months
Height: 107 cm
Weight: 18 kg
BMI calculation: 18 / (1.07 x 1.07) = 15.7
Plotting on the UK90 chart for girls aged 4 years 8 months, a BMI of 15.7 falls at approximately the 60th centile, which is firmly within the healthy weight range (2nd-90th centile). No action needed beyond continuing healthy habits.
Example 2: Year 6 Boy
Age: 10 years 9 months
Height: 145 cm
Weight: 45 kg
BMI calculation: 45 / (1.45 x 1.45) = 21.4
For a boy of this age, a BMI of 21.4 falls above the 91st centile, placing him in the overweight category. The NCMP letter would recommend speaking to the school nurse or GP. The focus should be on increasing physical activity and reducing sugary snacks, not calorie restriction.
Example 3: Teenager Going Through Puberty
Age: 13 years 2 months (girl)
Height: 160 cm
Weight: 52 kg
BMI calculation: 52 / (1.60 x 1.60) = 20.3
At 13, girls are typically in the midst of puberty, when body composition changes naturally. A BMI of 20.3 for a 13-year-old girl falls around the 65th centile - healthy weight. It is important not to apply adult standards; the same BMI of 20.3 would mean something quite different for an 8-year-old.
Common Mistakes and Tips
Frequently Asked Questions
At what age should I stop using child BMI centile charts?
The UK90 centile charts cover ages 0-23 years, but for practical purposes, adult BMI categories (18.5-24.9 healthy) are typically used from age 18 onwards. Between ages 16 and 18, there is a transition period where either system may be applied. Your GP can advise on which is most appropriate for your teenager.
My child was measured at school and I disagree with the result. What can I do?
You can request that your GP or health visitor re-measures your child. School measurements are conducted by trained staff but can occasionally be affected by heavy clothing, incorrect height measurement, or data recording errors. If the re-measurement confirms the result, ask for a referral to a paediatric dietitian or weight management service if appropriate.
Does ethnicity affect child BMI interpretation in the UK?
Yes. Children of South Asian heritage tend to have higher body fat percentages at lower BMI values, meaning the standard centile thresholds may underestimate health risk. Conversely, children of Black African or Caribbean heritage may have more lean muscle mass, and standard thresholds could overestimate risk. NICE guidelines acknowledge these differences, and clinicians should consider ethnicity alongside BMI when assessing health risk.
Can a child be too thin? When should I worry about underweight?
A child consistently below the 2nd centile warrants medical investigation to rule out underlying conditions such as coeliac disease, thyroid problems, or inadequate nutrition. However, some children are naturally slender and track just above the 2nd centile throughout childhood. As with overweight, the trajectory is key - a sudden drop across centile lines is more concerning than a child who has always been on a low centile.
UK Child Obesity Statistics 2024
| Age Group | Overweight % | Obese % |
|---|---|---|
| Reception (4-5 years) | 12.5% | 10.1% |
| Year 6 (10-11 years) | 14.6% | 23.4% |
Source: NHS National Child Measurement Programme 2022/23
UK Support Services
- NHS Healthier Families: Free recipes, tips, and activity ideas
- Change4Life: NHS programme with practical family advice
- School Nurses: Can provide confidential advice and referrals
- GP: Can refer to specialist services if needed
- HENRY: Local authority programmes for under-5s
Healthy Weight Tips by Age
| Age | Activity Guideline | Key Nutrition Focus |
|---|---|---|
| 2-4 years | 180 mins daily | Full-fat dairy, iron-rich foods |
| 5-11 years | 60+ mins moderate/vigorous daily | 5-a-day, limit sugar |
| 12-17 years | 60+ mins moderate/vigorous daily | Calcium, iron (especially girls) |