For children and teens, the same BMI means different things at different ages. BMI falls from age 2 to about 5 or 6, then rises through the teen years, and it differs between boys and girls. That is why a child’s BMI is compared with other children of the same age and sex. The result is a BMI-for-age percentile, and the categories are set by percentile, not by the adult cut-offs of 25 and 30.
How it is calculated
- BMI = weight in kg ÷ (height in meters)², or 703 × weight in lb ÷ (height in inches)². The formula is the same as for adults.
- The percentile comes from the growth reference for the child’s exact age in months and sex, using the LMS method that the CDC and the WHO publish with their charts.
- CDC categories (ages 2–19): underweight below the 5th percentile, healthy weight from the 5th to below the 85th, overweight from the 85th to below the 95th, obesity from the 95th. Severe obesity starts at 120% of the 95th percentile or a BMI of 35.
- WHO categories (ages 5–19): thinness below −2 standard deviations (SD), overweight above +1 SD, obesity above +2 SD.
Example
A 10-year-old boy who weighs 35 kg (77 lb) and is 140 cm (4 ft 7 in) tall has a BMI of 35 ÷ 1.4² = 17.9. On the CDC charts that is the 70th percentile: a healthy weight. For his age and height, healthy weight is 28 to 38 kg (62 to 83 lb). The same BMI of 17.9 in an adult would be underweight, which shows why the adult BMI calculator should not be used for children.
CDC or WHO?
Doctors in the US use the CDC charts from 2000. The WHO Growth Reference 2007 for ages 5 to 19 is used in many other countries; its overweight line (+1 SD) sits at about the 84th percentile, close to the CDC’s 85th. In the UK, the NHS calculator uses UK growth data from the Royal College of Paediatrics and Child Health, so the category for the same child can differ slightly. BMI-for-age charts start at age 2.
What BMI cannot tell you
BMI is a screening number. It does not measure body fat, and it cannot tell muscle from fat in an athletic teen. A child who is growing fast or entering puberty can change percentile quickly. A pediatrician looks at the trend over time, not a single number.
Frequently asked questions
What is a normal BMI for a child?
It depends on age and sex. A healthy BMI is anywhere from the 5th to below the 85th percentile. At age 10, that is a BMI of 14.2 to under 19.4 for boys and 14.0 to under 20.0 for girls. At 15, it is 16.6 to under 23.5 for boys and 16.3 to under 24.0 for girls. The charts on this page list every age from 2 to 19.
How do I calculate my child’s BMI?
Divide weight in kilograms by height in meters squared, or multiply weight in pounds by 703 and divide by height in inches squared. Then compare the result with the BMI-for-age chart for your child’s age and sex. The calculator above does both steps and gives the percentile.
What is overweight for a 10 year old?
For a 10-year-old boy, a BMI of 19.4 or more is overweight and 22.2 or more is obesity. For a 10-year-old girl, overweight starts at a BMI of 20.0 and obesity at 23.0. These are the 85th and 95th percentiles on the CDC charts.
What is a BMI in the 95th percentile?
It means the child’s BMI is higher than that of 95% of children of the same age and sex in the reference data. A BMI at or above the 95th percentile is classed as obesity. From 120% of the 95th percentile, it is severe obesity.
What’s the difference between BMI and BMI percentile?
BMI is the number from weight and height. The percentile shows how that BMI compares with other children of the same age and sex. Adults are judged by BMI alone, children by the percentile, because a healthy BMI changes as they grow.
Is the BMI calculator accurate for kids?
An adult BMI calculator is not, because adult categories do not apply to children. A calculator that uses BMI-for-age percentiles, like this one, gives the same screening result as the CDC or WHO charts. It is still a screening tool: it does not measure body fat directly.
When to worry about a child’s BMI?
Talk to your child’s doctor if the BMI is below the 5th percentile or at the 85th percentile or above, or if it moves up or down across the chart lines over a year or two. A single measurement is less important than the trend.
Sources
- CDC: Child and teen BMI categories
- CDC: Growth chart data files (BMI-for-age LMS parameters, bmiagerev.csv)
- WHO Growth Reference 5–19 years: BMI-for-age
- de Onis M et al. Development of a WHO growth reference for school-aged children and adolescents. Bull World Health Organ, 2007
- NHS: BMI calculator for children and teenagers
By Calcelate Team. Formula from the sources above.
- 2026-10-06 · Formula and texts checked
- 2007 · Reference data as of this date