BMI Calculator NHS Child
This independent UK child BMI tool calculates body mass index from height and weight for children and teenagers aged 2 to 17. A child’s BMI must then be interpreted by age- and sex-specific BMI centiles, so this page does not invent a centile from adult BMI cut-offs.
Child BMI calculator
What is a child BMI calculation?
BMI is weight in kilograms divided by height in metres squared. For children and young people, the raw BMI is not interpreted with adult cut-offs because body composition changes during growth. NHS and NICE guidance uses BMI centiles adjusted for age and sex.
How to use this calculator
- Enter the child’s age from 2 to 17.
- Select sex at birth.
- Enter height in centimetres and weight in kilograms.
- Select Calculate to see the raw BMI.
- Use the official NHS child BMI calculator to obtain the age- and sex-adjusted centile.
Results and interpretation
NICE recommends using appropriate age- and sex-adjusted charts for children and young people. Clinical classifications include overweight at the 91st BMI centile, clinical obesity at the 98th centile and severe obesity at the 99.6th centile. NHS public guidance describes a healthy-weight result as between the 2nd and 90th centile. A centile should be obtained from an appropriate UK child growth reference rather than inferred from the raw BMI alone.
Formula or methodology
BMI = weight (kg) ÷ height (m)². The calculator performs this arithmetic only. It deliberately does not manufacture a BMI centile because accurate child assessment requires the UK age- and sex-specific growth reference used by NHS/RCPCH tools.
Worked example
A child who weighs 35 kg and is 1.40 m tall has a raw BMI of 35 ÷ (1.40 × 1.40) = 17.9 kg/m². That number alone does not tell you whether the child is underweight, a healthy weight or above a healthy weight; age and sex are needed to determine the centile.
Who should use the official NHS child BMI service?
The NHS child BMI service is intended for children and teenagers aged 2 to 17 living in the UK. The NHS advises against using it for pregnancy, diagnosed or suspected eating disorders, or conditions that affect height or growth. For a child under 2, speak to a GP or health visitor about growth.
Limitations
BMI is a practical screening measure, not a diagnosis and not a direct measure of body fat or central adiposity. This independent tool calculates raw BMI only. If you are concerned about a child’s weight, growth or eating, contact a GP, school nurse or health visitor.
Authoritative sources
- NHS: BMI for children and teenagers
- NICE NG246: identifying and assessing overweight, obesity and central adiposity
- NHS: child healthy growth and weight
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FAQs
Can I use adult BMI ranges for a child?
No. Child BMI needs age- and sex-specific centile interpretation.
What age is the NHS child BMI calculator for?
The NHS service is for children and teenagers aged 2 to 17.
What information is needed for a child BMI assessment?
The NHS service uses date of birth, sex at birth, height, weight and the date the measurements were taken.
Is child BMI a diagnosis?
No. BMI is a screening estimate and should be interpreted alongside growth and clinical context.
Medical disclaimer: This independent calculator is for general information only and is not an NHS diagnostic service. If you are concerned about a child’s weight or growth, contact an appropriate healthcare professional.