BMI Calculator for Children UK | Kids BMI Percentile Checker
Check your child's BMI using NHS percentile charts. Free UK BMI calculator for children aged 2-18 with age and sex-specific results, percentile categories, and health guidance....
Last updated: February 2026
BMI Calculator for Children UK
Check your child's BMI percentile using NHS growth chart categories (ages 2-18)
Understanding BMI for Children in the UK
Body Mass Index (BMI) is a widely used measure for assessing whether a person is at a healthy weight for their height. However, calculating and interpreting BMI for children is fundamentally different from the way it works for adults. While adults are assessed against fixed BMI categories (such as underweight below 18.5, healthy weight 18.5 to 24.9, and so on), children require a more nuanced approach because their bodies are still growing and developing. A child's body composition changes significantly between the ages of 2 and 18, and boys and girls follow different growth trajectories. This is why the NHS and health professionals around the world use age-specific and sex-specific BMI percentile charts when assessing whether a child is at a healthy weight.
In the UK, the most common reference data used for children's BMI assessment is the British 1990 (UK90) growth reference, which was developed from measurements of thousands of British children. The NHS National Child Measurement Programme (NCMP) uses these charts to measure children in Reception year (aged 4 to 5) and Year 6 (aged 10 to 11) as part of a routine public health monitoring programme. Parents receive a letter with their child's results, which uses the percentile categories explained on this page. Our calculator above uses a simplified version of this approach, providing an approximate percentile category based on your child's age, sex, height, and weight.
Why Children's BMI Is Calculated Differently
The fundamental BMI formula is the same for children as for adults: weight in kilograms divided by height in metres squared. However, the way the result is interpreted is entirely different. For adults, a BMI of 25 always means overweight, regardless of whether the person is 20 or 60 years old. For children, the same BMI number can mean very different things depending on the child's age and sex.
Consider this example: a BMI of 18 would be considered a perfectly healthy weight for a 10-year-old boy, but it would put a 5-year-old boy above the 98th percentile (very overweight). This is because younger children typically have lower BMI values, and the expected BMI range increases as children grow. During puberty, both boys and girls experience significant changes: boys tend to develop more lean muscle mass, while girls tend to gain proportionally more body fat, particularly around the hips and thighs. These natural developmental differences mean that identical BMI numbers carry different meanings for boys and girls of the same age.
To account for these growth patterns, health professionals plot a child's BMI on percentile charts that show where the child falls relative to other children of the same age and sex. Rather than fixed cutoff numbers, the NHS uses percentile thresholds: the 2nd percentile (below which a child is considered underweight), the 91st percentile (above which a child is considered overweight), and the 98th percentile (above which a child is considered very overweight or obese).
NHS Growth Charts Explained
NHS growth charts are part of the Personal Child Health Record (often called the "Red Book") given to all parents in the UK when their baby is born. These charts track a child's growth over time and include separate curves for weight, height, head circumference, and BMI. For BMI specifically, the charts show curved lines representing different percentiles (2nd, 9th, 25th, 50th, 75th, 91st, and 98th), and a child's BMI is plotted against these lines to determine their weight category.
The percentile number represents the percentage of children of the same age and sex who have a lower BMI. For instance, if your child is at the 75th percentile, it means 75% of children their age and sex have a lower BMI, and 25% have a higher BMI. Being at the 50th percentile means the child is exactly average. The key NHS thresholds and their meanings are:
| Percentile Range | Category | What It Means |
|---|---|---|
| Below 2nd | Underweight | Fewer than 2 in 100 children of the same age and sex have a lower BMI. May need nutritional assessment. |
| 2nd to 91st | Healthy weight | This is the broad healthy range. Most children fall here. Continue balanced eating and active play. |
| 91st to 98th | Overweight | Only 2 to 9 in 100 children of the same age and sex have a higher BMI. Worth discussing with your GP. |
| Above 98th | Very overweight (obese) | Fewer than 2 in 100 children have a higher BMI. Clinical assessment and support recommended. |
Important: A single BMI measurement is just a snapshot. Growth trends over time are more informative than any single reading. If your child's BMI has been gradually increasing across percentile lines, or suddenly drops, that pattern is more significant than where they sit at one moment. This is why health visitors and GPs track growth over multiple visits.
When to See a GP About Your Child's Weight
It is natural to feel worried if your child's BMI appears outside the healthy range, but it is important not to panic. A BMI reading from an online calculator is an estimate and should not be treated as a diagnosis. However, there are certain situations where it is advisable to speak to your GP, health visitor, or school nurse:
BMI above the 98th percentile
If your child's BMI is consistently above the 98th percentile, a clinical assessment is recommended. Your GP can check for any underlying health conditions and refer you to a paediatric dietitian or weight management service if appropriate. The NHS offers free family-based weight management programmes in many areas.
BMI above the 91st percentile with other concerns
If your child is overweight and also has symptoms such as breathlessness during activity, joint pain, low self-esteem, or is being bullied about their weight, seek professional support. Early intervention is more effective and less distressing for the child.
BMI below the 2nd percentile
Underweight children may need investigation for underlying causes such as coeliac disease, food allergies, thyroid problems, or eating difficulties. Your GP can arrange appropriate tests and may refer to a paediatric dietitian for nutritional support.
Rapid changes in weight
If your child has gained or lost a significant amount of weight in a short period, or if they have crossed two or more percentile lines (up or down) on their growth chart, discuss this with your GP regardless of their current category.
Healthy Eating and Activity for Children: NHS and Change4Life Guidance
Whether your child's BMI is within the healthy range or not, establishing good eating habits and an active lifestyle in childhood sets the foundation for lifelong health. The NHS and the Change4Life campaign provide clear, evidence-based guidance for families. Here are the key recommendations:
Balanced Diet
- At least 5 portions of fruit and vegetables every day
- Starchy carbohydrates (bread, rice, potatoes, pasta) at each meal, choosing wholegrain where possible
- Some protein: beans, pulses, fish (2 portions per week including 1 oily), eggs, lean meat
- Some dairy or dairy alternatives for calcium
- Small amounts of unsaturated oils and spreads
- 6 to 8 glasses of water or sugar-free drinks daily
- Limit foods high in fat, salt, and sugar to occasional treats
Physical Activity
- Under 5s (walking): At least 180 minutes of activity daily, including energetic play
- 5 to 18 years: At least 60 minutes of moderate-to-vigorous activity every day
- Include activities that strengthen muscles and bones at least 3 days per week (climbing, jumping, gymnastics, swimming)
- Reduce sedentary screen time (aim for no more than 2 hours recreational screen time per day)
- Make it fun: cycling, swimming, football, dancing, playground games, walking to school
- The whole family getting active together is the most effective approach
Key principle: Never put a growing child on a "diet" or restrict their food intake without medical guidance. For overweight children, the NHS recommends focusing on slowing weight gain so the child can "grow into" their weight, rather than actively losing weight. Focus on the whole family making healthier choices together, and never single out the child or make them feel different.
BMI Limitations in Children
While BMI percentile charts are a useful screening tool, they have important limitations when applied to individual children. Understanding these limitations helps parents interpret results more accurately and avoid unnecessary worry.
Muscle Mass vs Fat
BMI cannot distinguish between muscle and fat. A child who is very physically active, plays sport regularly, or is naturally muscular may have a higher BMI without excess body fat. This is particularly common in children involved in swimming, gymnastics, rugby, martial arts, or athletics. A GP can assess body composition more accurately if needed.
Puberty Timing
Puberty can begin as early as age 8 in girls and age 9 in boys, or as late as age 13 to 14. Children who enter puberty early may temporarily appear to have a higher BMI percentile than their peers because they gain height and weight sooner. Conversely, late developers may appear lighter. Percentile charts use population averages and cannot account for individual variation in pubertal timing.
Growth Spurts
Children do not grow at a steady rate. They may gain weight before a height growth spurt, temporarily pushing their BMI higher. After the growth spurt, BMI often returns to a lower level. A single elevated reading during a pre-growth-spurt phase does not necessarily mean the child is overweight. This is why tracking BMI over time is more meaningful than a single measurement.
Ethnicity Differences
As with adults, health risks at any given BMI level can vary by ethnicity. Children from South Asian backgrounds may carry higher health risks at lower BMI levels, while children from Black African or Caribbean backgrounds may have higher muscle mass and bone density. The UK90 reference data was primarily developed from white British children, so results should be interpreted with caution for children from other ethnic backgrounds. Discuss this with your GP if relevant.
Frequently Asked Questions: Children's BMI
Why is BMI calculated differently for children?
Children's body fat changes as they grow, and boys and girls develop at different rates. Adult BMI categories do not apply. Instead, a child's BMI is compared against growth reference charts for children of the same age and sex, using percentile thresholds established by the NHS and WHO.
What BMI percentile is healthy for a child in the UK?
The NHS considers a healthy weight to be between the 2nd and 91st percentile. Below the 2nd is underweight, the 91st to 98th is overweight, and above the 98th is very overweight (obese). These thresholds are used consistently by the NHS National Child Measurement Programme.
At what age can you use BMI for children?
BMI-for-age percentile charts are designed for children aged 2 to 18 years. For children under 2, weight-for-length charts are used instead. After age 18, standard adult BMI categories apply. The NHS measures children routinely in Reception (age 4-5) and Year 6 (age 10-11).
Does puberty affect children's BMI?
Yes, puberty causes significant changes in body composition. Girls typically gain more body fat while boys gain more muscle mass. Early or late developers may temporarily appear outside the healthy range. BMI percentile charts account for average growth patterns, but individual variation in pubertal timing means a GP assessment may be needed for borderline results.
Can a muscular or sporty child have a misleadingly high BMI?
Yes. BMI cannot distinguish between muscle and fat. Children who are very active in sport, particularly muscle-building activities like swimming, gymnastics, or rugby, may register a higher BMI percentile without having excess body fat. If your child is fit, active, and eating well, a slightly elevated BMI may not be a concern. Your GP can provide a more accurate assessment.
What should I do if the NCMP letter says my child is overweight?
The NCMP letter is an invitation to take action, not a cause for alarm. Avoid putting your child on a "diet" or making them feel singled out. Instead, focus on the whole family making gradual, positive changes: eating more fruit and vegetables, being active together, reducing sugary drinks and snacks, and limiting screen time. The NHS Change4Life website offers free recipes, activity ideas, and support. If the result says "very overweight," book a GP appointment for further guidance.
Is this calculator as accurate as the NHS one?
This calculator provides an approximate percentile category using simplified reference data based on WHO and UK growth standards. For the most accurate clinical assessment, use the official NHS BMI calculator for children at nhs.uk, or speak to your GP or health visitor who can plot your child's measurements on the full UK90 growth charts.
Written by UKCalculator.com
Reviewed by: UK Calculator, Founder & Developer - UKCalculator.com
Credentials: Founder & Developer of UKCalculator.com
Clinical Focus: Childhood obesity prevention, growth and development assessment, family-based weight management. Contributor to NHS England child health guidance.
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Expert Reviewed — This calculator uses NHS growth chart percentile categories and is reviewed by our team of health experts. Last verified: February 2026.
Last updated: February 2026 | Based on NHS/WHO growth reference data
Tips for Accurate Measurements
- Measure height without shoes, standing straight against a wall
- Weigh your child in light clothing or underwear
- Measure at the same time of day for consistency
- Use the same scales each time
- For young children, your health visitor can measure accurately
Understanding Percentile Results
Key things to know about percentile results:
- Healthy range is broad - 2nd to 91st percentile is all healthy
- One reading is not enough - trends over time matter more
- BMI is a screening tool - not a diagnosis
- See your GP if you are concerned about any result
Common Questions
Is this calculator free?
Yes, all our calculators are 100% free to use with no registration required.
Should I put my child on a diet?
Never restrict a child's food without medical guidance. Focus on healthy family meals and active play instead.
Can I use this for children under 2?
No, this calculator is for ages 2-18 only. For younger children, speak to your health visitor.
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