Child BMI Calculator
Calculate your child’s BMI and healthy weight range with age- and gender-adjusted results. Suitable for children and teenagers aged 2 to 18 years.
Child BMI & Healthy Weight Estimator
Age- and gender-adjusted results using UK NHS centile reference data
BMI healthy ranges differ slightly between boys and girls due to growth patterns.
Enter age in whole years, between 2 and 18.
Measure without shoes, standing straight against a wall.
Measure in light clothing, without shoes.
BMI Result
Age- & Gender-Adjusted Child BMI
Enter your child’s details and click Calculate BMI to see their BMI, centile, and healthy weight range.
Healthy BMI Ranges by Age & Sex
UK NHS healthy BMI ranges for children and teenagers by age and sex. These are based on the UK 1990 growth reference charts (UK90). A child’s BMI is considered healthy if it falls between the 2nd and 91st centile for their age and sex.
| Age | Boys Healthy BMI Range | Girls Healthy BMI Range | Overweight (Boys) | Overweight (Girls) |
|---|---|---|---|---|
| 5 years | 13.5 – 17.4 | 13.2 – 17.4 | ≥ 17.4 | ≥ 17.4 |
| 7 years | 13.8 – 18.5 | 13.5 – 18.3 | ≥ 18.5 | ≥ 18.3 |
| 9 years | 14.2 – 20.1 | 14.0 – 19.8 | ≥ 20.1 | ≥ 19.8 |
| 11 years | 14.8 – 21.7 | 14.7 – 21.8 | ≥ 21.7 | ≥ 21.8 |
| 13 years | 15.7 – 23.4 | 15.8 – 23.7 | ≥ 23.4 | ≥ 23.7 |
| 15 years | 16.8 – 24.9 | 17.0 – 25.3 | ≥ 24.9 | ≥ 25.3 |
| 17 years | 17.8 – 26.0 | 17.9 – 26.6 | ≥ 26.0 | ≥ 26.6 |
Child BMI FAQ
Everything UK parents need to know about child BMI, healthy weight ranges, and what to do if you have concerns.
BMI for children is calculated using the same formula as adults: weight in kilograms divided by height in metres squared. However, for children and teenagers aged 2 to 18, the result must be compared against age- and sex-specific growth charts rather than fixed adult cut-off values, because a healthy BMI range changes as children grow.
In the UK, the NHS uses BMI centile charts to assess a child’s weight. A healthy weight is defined as a BMI between the 2nd and 91st centile for the child’s age and sex. A BMI at or above the 91st centile is considered overweight, and at or above the 98th centile is considered very overweight (obese). A BMI below the 2nd centile is considered underweight.
No. Although the BMI formula (weight ÷ height²) is the same, adult BMI uses fixed cut-off points (e.g. 18.5 to 24.9 for healthy weight) that do not apply to children. Children’s bodies change rapidly as they grow, so a child’s BMI score must be interpreted using age- and gender-specific growth centile charts. This is why a dedicated child BMI calculator is needed.
Child BMI calculators are suitable for children aged 2 to 18 years. For children under 2, weight-for-length measurements are used instead of BMI. Once a child turns 18, adult BMI categories and cut-off values apply.
If your child’s BMI falls above the 91st centile, it is advisable to speak with your GP or a paediatric dietitian. They can assess your child’s weight in the context of their overall health, growth trajectory, and family history. Avoid putting children on restrictive diets without medical guidance. Focus instead on encouraging regular physical activity, balanced meals, and healthy family habits.
BMI is a useful screening tool but not a complete measure of health. It does not account for muscle mass, bone density, or how weight is distributed. A child who is very athletic or muscular may have a higher BMI without excess body fat. BMI results should always be discussed with a healthcare professional, who can consider the full picture of a child’s health and growth.
Online child BMI calculators provide a useful estimate based on the standard BMI formula and UK NHS centile reference data, but they are not a substitute for a clinical assessment. Accuracy depends on entering precise height and weight measurements. For the most reliable results, use professionally measured height and weight, such as those taken at a GP or school health check.