How Is BMI Calculated for Children in the UK?
Child BMI is calculated using exactly the same formula as adult BMI — weight in kilograms divided by height in metres squared. However, the way the result is interpreted is completely different. For adults, fixed ranges (18.5–24.9 for healthy weight) apply to everyone. For children, BMI must be interpreted using age and sex-specific centile charts, because children's body composition changes substantially as they grow.
The NHS uses the UK90 growth reference charts, developed from a large representative sample of UK children measured in 1990. These charts allow a child's BMI to be expressed as a centile — telling parents how their child's BMI compares to other UK children of the same age and sex. A child at the 50th centile has a BMI equal to the average for their age and sex; a child at the 91st centile has a BMI higher than 91% of children their age.
Key point for parents: A BMI number alone means nothing for children. A BMI of 18 might be perfectly healthy for a 15-year-old but unusually high for a 7-year-old. Always interpret your child's BMI in the context of their age, sex, and centile position — ideally with help from your GP or health visitor.
What Is a Healthy BMI Centile for a Child?
The NHS defines a healthy weight for children as a BMI between the 2nd and 91st centile for their age and sex. This broad range reflects the normal variation in children's growth and development. Most children — around 89% — fall within this range.
It is important to understand that being at the higher end of the healthy range is not concerning, and that centiles naturally fluctuate as children grow. A child who has always been at the 75th centile is not at risk just because their centile is relatively high — consistency over time is what matters, not a single reading. What raises concern is when a child's centile is consistently above the 91st, or when their centile is rising rapidly over time.
What Does Overweight Mean for a Child in the UK?
The NHS classifies a child as overweight if their BMI is at or above the 91st centile for their age and sex. This means their BMI is higher than 91% of UK children of the same age and sex. A child is classified as very overweight (clinically obese) if their BMI is at or above the 98th centile.
Childhood obesity in the UK has become a significant public health concern. According to NHS England data, approximately 1 in 5 children in Reception (aged 4–5) and 1 in 3 children in Year 6 (aged 10–11) are overweight or obese. Children who are overweight are more likely to carry excess weight into adulthood, with associated health risks including type 2 diabetes, cardiovascular disease, and joint problems.
However, it is crucial that parents approach this topic carefully. Children should never be put on restrictive diets without medical supervision, and should not be made to feel ashamed about their weight. The focus should always be on whole-family healthy habits rather than individual weight targets.
What Should I Do If My Child's BMI Is High?
If the calculator suggests your child's BMI is in the overweight or very overweight range, the most important first step is to speak to your GP or health visitor. They will carry out a proper assessment using your child's growth charts, review their development over time, and advise on appropriate next steps.
Do not attempt to put your child on a diet without professional guidance. Restricting calories in children can interfere with growth, development, and the development of a healthy relationship with food. Instead, the NHS recommends a whole-family approach focused on:
- Balanced, nutritious meals: More fruit, vegetables, wholegrains, and lean protein. Less ultra-processed food, sugary drinks, and excessive portion sizes
- Active play and exercise: Children aged 5–17 should aim for at least 60 minutes of moderate to vigorous physical activity every day
- Reducing sedentary screen time: Particularly for younger children — aim for no more than 2 hours of recreational screen time per day outside of schoolwork
- Good sleep habits: Children need more sleep than adults — primary school children need 9–11 hours, secondary school children need 8–10 hours
- Family meals together: Eating as a family reduces the likelihood of overeating and helps children develop healthy attitudes to food
Your GP can refer your child to local NHS children's weight management services, which typically involve the whole family and use a supportive, non-stigmatising approach.
Never comment negatively on a child's weight or body. Research shows that weight stigma in childhood — including from parents and carers — is associated with disordered eating, lower self-esteem, and poorer mental health outcomes. Focus on healthy habits, not weight or BMI numbers.
What If My Child's BMI Is Too Low?
If your child's BMI is below the 2nd centile, this is classified as underweight and should also be discussed with your GP or health visitor. Being underweight in childhood can affect growth, bone development, immune function, concentration, and energy levels.
There are many potential causes of low BMI in children, including insufficient calorie intake, food aversion or selective eating, underlying medical conditions such as coeliac disease, Crohn's disease, or thyroid disorders, as well as psychological factors. Your GP will carry out appropriate investigations and may refer to a paediatric dietitian or specialist.
National Child Measurement Programme (NCMP)
In England, all children have their height and weight measured in Reception (aged 4–5) and Year 6 (aged 10–11) as part of the NHS National Child Measurement Programme. Parents receive a letter with their child's results and are offered support if their child is outside the healthy weight range.
This programme provides valuable population-level data on childhood obesity trends and ensures children at risk are identified early. If you received a letter following an NCMP measurement and are concerned, your first step should be to speak to your GP or health visitor.
NHS Support for Families: Your GP can refer your child to local NHS children's weight management services. The NHS Healthier Families website also offers free tools, meal ideas, and activity suggestions specifically designed for UK families with children.