👧 NHS Age & Sex Specific · UK-WHO Reference Data · Updated July 2026
Child BMI Calculator UK — NHS Children's BMI Calculator
Calculate your child's BMI centile for ages 2–18 using NHS age and sex-specific percentile charts. Free, instant, aligned with UK-WHO growth data and NCMP thresholds. No sign-up needed.
⬇ Underweight Below 2nd centile
✅ Healthy Weight 2nd – 91st centile
⚠ Overweight 91st – 98th centile
🔴 Obese Above 98th centile
📅 Last Updated: July 2026🩺 NHS NCMP Aligned✅ Medically Reviewed🆓 Free · Ages 2–18
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Written byBMI Calculator NHS Editorial TeamPaediatric Health Writers specialising in NHS child growth standards, BMI centile interpretation, and children's weight guidance for UK parents
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Medically ReviewedNHS-Aligned Health Review PanelReviewed against NHS NCMP guidance, RCPCH UK-WHO growth chart standards, and NICE children's weight management guidelines · June 2026
Child BMI Calculator UK — What Is It and How Does It Work?
Our free child BMI calculator UK — also called the NHS children's BMI calculator — calculates your child's Body Mass Index and instantly converts it to an age- and sex-specific centile using UK-WHO NHS reference data. This centile shows where your child sits relative to other UK children of exactly the same age and sex.
This is fundamentally different from adult BMI calculation. For adults, the NHS uses fixed thresholds — BMI 18.5–24.9 for healthy weight. For children, these thresholds do not apply. A BMI of 17 might be perfectly healthy for a 6-year-old girl and slightly underweight for a 16-year-old teenager. This is why the NHS uses age and sex-specific percentile charts to interpret children's BMI — and this calculator handles that conversion automatically for you.
The NHS calls this approach the "BMI children age and sex percentile" system, used in the National Child Measurement Programme (NCMP), GP practices, and health visitor checks across England. Our calculator uses the same reference dataset and the same centile thresholds as the NCMP.
Key Difference: Adult BMI uses fixed thresholds (18.5, 25, 30). Children's BMI uses centiles — percentile rankings compared to other UK children the same age and sex. The 2nd, 91st, and 98th centile lines are the NHS boundaries for underweight, overweight, and obese in children. Our bmi calculator child tool calculates this automatically.
How the Children's BMI Calculator Works
The children's BMI calculator follows these steps:
Step 1 — BMI Calculation: Weight (kg) ÷ Height (m)² = BMI (same formula as adults)
Step 2 — Age & Sex Lookup: The child's exact age in months and sex is matched against UK-WHO LMS reference parameters
Step 3 — Z-Score Calculation: The LMS formula converts the BMI to a z-score: z = [(BMI/M)^L − 1] ÷ (L × S)
Step 4 — Centile Conversion: The z-score is converted to a centile (0–100) using the standard normal distribution
Step 5 — NHS Category: The centile is compared to NHS NCMP thresholds to give a category result
This is the same clinically validated process used by NHS Digital's digital growth charts API and by RCPCH's official growth chart application. Our bmi calculator for children above applies all five steps automatically — you just enter age, sex, height, and weight.
These are the same thresholds used in the NHS National Child Measurement Programme (NCMP) — the annual scheme that measures BMI in Reception (ages 4–5) and Year 6 (ages 10–11) across England. If your child has received an NCMP result letter, the categories match exactly.
Average Weight for Children UK — NHS Reference by Age 2026
One of the most searched queries by UK parents is "average weight for 10 year old boy UK" and similar age-based questions. The table below shows the NHS 50th centile (UK average) BMI and approximate weight for common ages, for both boys and girls:
Age
Boys — Avg BMI
Boys — Avg Weight*
Girls — Avg BMI
Girls — Avg Weight*
2 years
16.4
~12 kg
16.1
~11.5 kg
4 years
15.5
~16 kg
15.3
~15.5 kg
6 years
15.2
~20 kg
15.1
~19.5 kg
8 years
15.8
~25 kg
15.8
~25 kg
10 years
17.0
~32–34 kg
17.2
~33–35 kg
12 years
18.5
~42 kg
19.0
~44 kg
14 years
20.5
~53 kg
21.0
~54 kg
16 years
22.0
~62 kg
22.5
~61 kg
18 years
22.5
~68 kg
22.5
~64 kg
*Average weights are approximate, calculated at average height for that age. Individual variation is wide and completely normal. Do not compare your child to these averages — use the child BMI calculator above for an age-accurate centile result specific to your child's measurements.
Average Weight for a 10 Year Old Boy UK — Detailed
The average weight for a 10 year old boy in the UK (50th centile) is approximately 32–34 kg, at an average height of approximately 137–140 cm. This gives an average BMI of approximately 16.5–17.5 for a 10-year-old UK boy. However, the healthy weight range (2nd–91st centile) for a 10-year-old boy at 137 cm is approximately 23–42 kg — a very wide range that reflects normal variation in body size at this age.
At age 10, UK boys are entering a period of variable pre-pubertal growth. Some boys start their pubertal growth spurt as early as 9–10, while others don't begin until 12–13. This is why comparing a 10-year-old boy's weight to an "average" figure can be misleading — centile-based assessment using our children's BMI calculator UK gives a much more meaningful result.
Important: The "average" UK child weight is not the same as the "healthy" weight. Approximately 30% of 10–11 year olds in England are overweight or obese on NHS NCMP data (2023–24). This means the average UK child weight is slightly above the 50th centile healthy target. Always use the centile system, not national averages, to assess your child's weight.
Child BMI for Infants and Babies — Under 2 Years
A common question is whether this tool works as a BMI calculator for infants or a baby BMI calculator. The answer is that for children under 2 years, the NHS does not use BMI at all. Instead, the NHS uses weight-for-length charts — where a baby's weight is compared against other babies of the same length, not the same age as in BMI calculations.
This distinction exists because in the first two years, rapid changes in body proportions make BMI an unreliable indicator. Our baby weight percentile calculator UK provides the correct weight-for-length centile for children from birth to 2 years using UK-WHO reference data. For the full 0–18 age range including both weight and height centiles, use our child growth chart calculator UK.
🏥Aligned with NCMP — same thresholds as NHS school measurements
👶Under 2: use our baby weight percentile calculator instead
⚠️Adult BMI range (18.5–24.9) does NOT apply to children
What to Do Based on Your Child's BMI Result
Once you have your child's BMI centile from the NHS child BMI calculator above, here is how to act on each category in line with NHS 2026 guidance:
✅ 2nd–91st Centile — Healthy Weight
Your child's BMI centile is in the NHS healthy range. Continue to support a balanced diet based on the NHS Eatwell Guide, regular physical activity (at least 60 minutes daily for school-age children), and adequate sleep. No weight management action is needed. Recheck annually or at the next NHS health check. Use our child growth chart calculator UK to track height and weight centiles over time.
⚠️ 91st–98th Centile — Overweight
Speak to your GP or health visitor — they will look at the growth trend over time rather than a single measurement. The NHS approach to children's weight is positive and family-centred, not focused on restriction. Key NHS recommendations include: reducing sugary drinks and ultra-processed foods, increasing active play, limiting recreational screen time, and eating family meals together. NHS referral to a paediatric dietitian or healthy families programme may be available in your area.
🔴 Above 98th Centile — Obese
Please speak to your GP. NHS children's weight management programmes are available across England, including dietitian support, physical activity referrals, and behavioural support for the whole family. The NHS approach is non-stigmatising and evidence-based. Do not put children on restrictive diets without professional guidance — this can harm children's relationship with food and their long-term mental health.
⬇ Below 2nd Centile — Underweight
Speak to your GP or health visitor promptly. A centile below the 2nd may indicate insufficient calorie intake, growth issues, coeliac disease, or another underlying cause. Do not attempt to increase your child's weight without professional guidance. Use our percentile calculator UK to check height and weight centiles separately for additional context.
⚠️ Important: A single BMI centile reading should never be used to label a child as overweight or underweight. The NHS assesses trends over time — a child consistently on the 92nd centile who has always tracked at this level may simply be naturally large-framed. A child who has moved from the 50th to the 91st centile in 12 months is a more meaningful concern. Always discuss results with your GP or health visitor.
NHS Tips for Healthy Child Weight — 2026
Family meals: Eating together reduces children's consumption of ultra-processed foods and models healthy eating habits
Active play: Children aged 5–18 need at least 60 minutes of moderate-to-vigorous activity daily (NHS 2026)
Limit screen time: Recreational screen time should be limited — active play and outdoor activity encouraged
No sugary drinks: Replace fizzy drinks and fruit juice with water or milk — the biggest source of excess child calories
Sleep: Ages 6–12 need 9–12 hours; teens 13–18 need 8–10 hours — poor sleep increases appetite hormones
Positive body language: Avoid weight-focused comments around children — focus on energy and health, not size
NICE — Weight Management: Lifestyle Services for Overweight or Obese Children and Young People (PH47)
⚠️ Medical Disclaimer: This child BMI calculator NHS provides approximate centile estimates based on UK-WHO growth chart reference data. Results are for educational and awareness purposes only — not clinical diagnosis. Child BMI centile must be interpreted by a qualified healthcare professional (GP, health visitor, or paediatrician) alongside growth trend data and full clinical assessment. BMI Calculator NHS is not affiliated with NHS England, RCPCH, or any government body. Always consult your GP or health visitor for concerns about your child's growth or weight. See our Disclaimer, Privacy Policy, and Terms of Service.
Frequently Asked Questions — Child BMI Calculator UK
The NHS calculates BMI for children using the same formula as adults — weight (kg) ÷ height (m)². However, the result is then converted to an age and sex-specific centile using UK-WHO growth chart reference data. This centile shows how the child's BMI compares to other UK children of exactly the same age and sex. Adult BMI thresholds (18.5–24.9) do not apply to children — the NHS uses 2nd, 91st, and 98th centile as category boundaries instead.
According to NHS guidance, a BMI centile between the 2nd and 91st centile is considered healthy for most UK children aged 2–18. A centile at or above the 91st centile indicates overweight; at or above the 98th indicates obese; below the 2nd may indicate underweight. These are the same thresholds used in the National Child Measurement Programme (NCMP). For full centile tracking across height and weight, use our child growth chart calculator UK.
The average weight for a 10 year old boy in the UK (50th centile) is approximately 32–34 kg, at an average height of 137–140 cm. The healthy weight range (2nd–91st centile) for a 10-year-old boy at 137 cm is approximately 23–42 kg. However, comparing to averages is less meaningful than using a centile — use our child BMI calculator UK above for an accurate age-specific result. Remember: approximately 30% of Year 6 boys in England are overweight or obese, so "average" weight is above ideal.
This child BMI calculator NHS covers ages 2–18 years. For babies and infants under 2 years, the NHS does not use BMI — it uses weight-for-length charts instead. Use our baby weight percentile calculator UK for children from birth to 2 years, which applies UK-WHO weight-for-length reference data. The full 0–18 range is covered by our child growth chart calculator UK.
Child BMI centile is a useful population screening tool used by the NHS but does not directly measure body fat. It can be influenced by muscle mass, bone density, and pubertal stage. The NHS uses BMI centile as one data point alongside growth trends over time, clinical assessment, and other measurements. A single centile reading should never be used to label a child. Always consult your GP or health visitor for professional interpretation. Our percentile calculator UK gives additional height and weight centile context.
The NHS BMI children age and sex percentile system compares a child's BMI to UK-WHO reference data for children of exactly the same age and sex. Because healthy BMI values change significantly as children grow — and differ between boys and girls — the NHS uses separate centile charts rather than fixed thresholds. The centile result (0.4th to 99.6th) shows what percentage of UK children have a lower BMI. This is the same system used in the National Child Measurement Programme (NCMP) and NHS Red Book growth charts.
This children's BMI calculator covers ages 2 to 18 years (24–216 months). For children under 2, use our baby weight percentile calculator UK. At age 18, adults transition to adult BMI thresholds (18.5–24.9 for healthy weight) — check your BMI with our BMI Calculator NHS.
If your child's BMI centile is consistently at or above the 91st centile, speak to your GP or health visitor. They will assess the growth trend over time and may refer to a paediatric dietitian or NHS children's weight management programme. The NHS approach is positive and family-centred. Do not put children on restrictive diets without professional guidance. Focus on healthy family habits: active play, balanced meals, limiting sugary drinks, and adequate sleep. See our child growth percentiles explained guide for more context.
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