BMI for Children and Teens: Why Percentiles Are Used Instead of Adult Ranges
If you enter a child's height and weight into an adult BMI formula, you get a number, but the adult categories that go with it would be misleading. Children's body composition changes as they grow, and differently for girls and boys. That is why health services compare a child's BMI with that of other children of the same age and sex, and express it as a percentile. This page explains how that works, and why our calculator sends families to the official children's tools instead of computing percentiles itself.
Why adult categories do not work for children
Adults have finished growing, so a single set of cut-offs can be applied to all of them. Children have not. The amount of body fat changes with age during childhood, rising and falling at different points, and it differs between girls and boys, especially around puberty. A BMI of 20 might be well within the expected range for a 16-year-old and well above it for a 7-year-old.
The CDC puts it plainly: because children and teens are still growing, their BMI is compared with others of the same sex and age. It describes BMI percentiles as the most commonly used clinical indicator to assess growth patterns in children. Race is not a factor in the calculation.
What a percentile means
A percentile places a child within a reference population. A girl whose BMI is at the 60th percentile for her age has a BMI equal to or higher than 60% of girls of the same age in the reference data. It is a comparison with the reference, not a grade. The percentile is read from growth charts built from large measured samples; the calculation uses statistical parameters published for every month of age, which is why it needs the child's exact age and sex.
That is also why our calculator does not try to produce a percentile. Doing it properly means implementing the official reference tables and method exactly, including the CDC's extended charts for very high BMIs. The official tools already do this, and they come with advice from the organisations that publish them.
The CDC categories
| Percentile | Category |
|---|---|
| Below the 5th | Underweight |
| 5th to below the 85th | Healthy weight |
| 85th to below the 95th | Overweight |
| 95th or above | Obesity |
| 120% of the 95th percentile or more, or a BMI of 35 or more | Severe obesity |
The CDC also describes an expanded definition of severe obesity, following the American Academy of Pediatrics: class 2 from 120% to below 140% of the 95th percentile (or a BMI of 35 to below 40) and class 3 from 140% of the 95th percentile (or a BMI of 40 or more). These finer categories exist because the traditional charts were not designed to describe very high BMIs precisely.
How other organisations do it
The World Health Organization uses a related approach based on standard deviations rather than percentiles. For children and adolescents aged 5 to 19, overweight is a BMI-for-age more than one standard deviation above the median of the WHO Growth Reference, and obesity is more than two standard deviations above it. For children under 5, the WHO uses weight-for-height and its Child Growth Standards.
In the UK, the NHS offers a BMI calculator for children and teenagers aged 2 to 17, which gives a centile and explains the result. NICE's guideline on overweight and obesity also suggests that clinicians consider measuring waist-to-height ratio in children aged 5 and over, using the NHS calculator for that age group. Adult waist bands, described in our waist-to-height ratio guide, should not be applied to children.
Because the references and cut-offs differ, the same child can be described slightly differently by different tools. That is one more reason to rely on the calculator used by your own health service and to discuss results with a professional.
Which calculator to use
- United States: the CDC's BMI calculator for children and teens (ages 2 to 19), on cdc.gov.
- United Kingdom: the NHS BMI calculator for children and teenagers (ages 2 to 17), on nhs.uk.
- Elsewhere: your national health service or paediatrician, who may use WHO references.
For ages 18 and 19, the NHS uses adult categories while the CDC still uses percentiles. Either way, a single measurement is a snapshot. Paediatricians and school nurses look at how a child is growing over time, along their own curve, rather than at one number.
Talking about weight with children and teenagers
Weight can be a sensitive subject for young people, and comments from adults can stick. NICE asks health professionals to seek permission from the family and the child before discussing weight or taking measurements, and to keep discussions sensitive and non-judgemental. Parents can borrow the same approach:
- Talk about health, energy, strength and enjoyment rather than size or appearance.
- Make changes as a family, such as more vegetables at meals, water instead of sugary drinks and active time together, rather than singling out one child.
- Avoid putting a child on a restrictive diet without professional advice; growing bodies have specific needs.
- Watch for signs that food or weight are causing distress, such as skipping meals, secrecy around eating or strong anxiety about body shape, and seek help early from a GP or paediatrician.
For the adult side of the picture, see adult BMI categories and what is a healthy weight?
Sources
- Centers for Disease Control and Prevention, "Child and Teen BMI Categories".
- World Health Organization, "Obesity and overweight" fact sheet: definitions for children under 5 and aged 5 to 19.
- NHS, "Calculate your body mass index (BMI) for children and teenagers".
- NICE guideline NG246, "Overweight and obesity management", recommendations 1.10.1 to 1.10.5 (nice.org.uk).
Healthy weight & BMIWhat is a healthy weight?
Why health services use BMI to describe a healthy weight, what it misses, which other measures help and what affects weight beyond food and exercise.
Healthy weight & BMIAdult BMI categories explained
Underweight, healthy weight, overweight and the three obesity classes, the lower NICE thresholds for some ethnic groups, and how to read your number.
Healthy weight & BMIWaist-to-height ratio
Keep your waist to less than half your height: how to measure, how to calculate the ratio and what the NICE bands mean.