A worked example
A girl aged 8 years and 6 months, 130 cm tall and weighing 27 kg, has a BMI of 16.0. That’s higher than about 52.2% of girls her age on the WHO curve, which counts as healthy weight. At her height, a healthy weight is 22.0 to 30.4 kg.
Why children need their own BMI chart
BMI is weight divided by height squared for everyone, but for a child the same number means different things at different ages. It falls through the toddler years, bottoms out around five or six, then climbs through puberty, and girls and boys climb differently. A BMI of 18 is higher than 93% of six-year-old girls on the WHO curve, and higher than only 14% of sixteen-year-old girls.
So a child’s BMI is read against children of the same age and sex. This calculator uses WHO’s curves: the Child Growth Standards up to five years, and the growth reference for school-age children and adolescents from five to nineteen. It reads them month by month, in the same way as WHO’s own software.
The WHO cut-offs
The categories are set in standard deviations from the median child. −2 SD is about the 2nd percentile, +1 SD the 84th, +2 SD the 98th and +3 SD the 99.9th. Under five, WHO is more careful about the word overweight: above +1 SD is only a possible risk of it, and the overweight and obesity lines sit one step higher.
From five years on, the reference was built so that the lines meet the adult ones: at nineteen, +1 SD sits at a BMI of about 25 and +2 SD at about 30, which is where adult overweight and obesity begin.
| Category | 2 to 4 years | 5 to 19 years |
|---|---|---|
| Severe thinness | below −3 | below −3 |
| Thinness | −3 to −2 | −3 to −2 |
| Healthy weight | −2 to +1 | −2 to +1 |
| Possible risk of overweight | +1 to +2 | – |
| Overweight | +2 to +3 | +1 to +2 |
| Obesity | above +3 | above +2 |
BMI cut-offs by age for girls
The BMI at each line, at every birthday from two to eighteen. Between birthdays the lines move a little each month, which the calculator takes into account.
| Age (years) | Thinness below | Overweight above | Obesity above |
|---|---|---|---|
| 2 | 13.3 | 18.7 | 20.6 |
| 3 | 13.1 | 18.4 | 20.3 |
| 4 | 12.8 | 18.5 | 20.6 |
| 5 | 12.8 | 16.9 | 18.8 |
| 6 | 12.7 | 17.0 | 19.2 |
| 7 | 12.7 | 17.3 | 19.8 |
| 8 | 12.9 | 17.7 | 20.6 |
| 9 | 13.1 | 18.3 | 21.5 |
| 10 | 13.5 | 19.0 | 22.6 |
| 11 | 13.9 | 19.9 | 23.7 |
| 12 | 14.4 | 20.8 | 25.0 |
| 13 | 14.9 | 21.8 | 26.2 |
| 14 | 15.4 | 22.7 | 27.3 |
| 15 | 15.9 | 23.5 | 28.2 |
| 16 | 16.2 | 24.1 | 28.9 |
| 17 | 16.4 | 24.5 | 29.3 |
| 18 | 16.4 | 24.8 | 29.5 |
BMI cut-offs by age for boys
The same lines for boys. Up to five, the overweight and obesity columns are WHO’s under-five lines at +2 and +3 SD.
| Age (years) | Thinness below | Overweight above | Obesity above |
|---|---|---|---|
| 2 | 13.8 | 18.9 | 20.6 |
| 3 | 13.4 | 18.4 | 20.0 |
| 4 | 13.1 | 18.2 | 19.9 |
| 5 | 13.0 | 16.6 | 18.2 |
| 6 | 13.0 | 16.8 | 18.5 |
| 7 | 13.1 | 17.0 | 19.0 |
| 8 | 13.3 | 17.4 | 19.7 |
| 9 | 13.5 | 17.9 | 20.5 |
| 10 | 13.7 | 18.5 | 21.4 |
| 11 | 14.1 | 19.2 | 22.5 |
| 12 | 14.5 | 19.9 | 23.6 |
| 13 | 14.9 | 20.8 | 24.8 |
| 14 | 15.5 | 21.8 | 25.9 |
| 15 | 16.0 | 22.7 | 27.0 |
| 16 | 16.5 | 23.5 | 27.9 |
| 17 | 16.9 | 24.3 | 28.6 |
| 18 | 17.3 | 24.9 | 29.2 |
WHO, IOTF or your country’s own chart?
Not every country uses WHO’s curves. Germany’s paediatric guidelines use the Kromeyer-Hauschild charts, the UK uses its UK90 charts, and much research uses the IOTF cut-offs, which are built to meet BMI 25 and 30 at eighteen. They mostly agree, but a child close to a line can land on different sides of it depending on the chart. If your child’s doctor or school nurse uses a different chart, theirs is the one to go by.
One reading is a snapshot
BMI can’t tell muscle from fat, and children who start puberty early or late can sit high or low on the curve for a while. What tells you more is the line a child follows over several years. A child who has tracked the same percentile since nursery is in a very different place from one who has crossed two lines in a year. If a result worries you, a doctor can look at that curve with you.
Questions people ask
Why not use the adult BMI for a teenager?
Because teenagers are still growing, and their BMI rises with age even when nothing is wrong. The adult lines of 25 and 30 only apply from 19, which is where the WHO curves meet them. For 19 and over, use the adult BMI calculator.
What percentile is a healthy weight?
On the WHO curves, roughly the 2nd to the 84th percentile, which is −2 to +1 SD. American charts use the CDC’s lines instead: under the 5th percentile is underweight, 85th to 95th overweight and 95th or above obesity. Same idea, slightly different lines, so check which one a number came from.
Does it work for babies and toddlers?
It starts at two years. Under two, children are measured lying down and checked on weight-for-length charts instead, which your child’s health records or growth booklet will have.
My child is sporty and muscular. Is BMI still useful?
It can overstate fatness in muscular children, the same as in adults. Waist size adds the missing part: a long UK study supported the simple rule of keeping the waist under half of height for children too, which the waist-to-height calculator checks.
Sources
- de Onis M, et al. Development of a WHO growth reference for school-aged children and adolescents. Bull World Health Organ, 2007
- WHO Multicentre Growth Reference Study Group. WHO Child Growth Standards based on length/height, weight and age. Acta Paediatr Suppl, 2006
- World Health Organization. BMI-for-age (5-19 years): growth reference data and cut-offs
- Cole TJ, Lobstein T. Extended international (IOTF) body mass index cut-offs for thinness, overweight and obesity. Pediatr Obes, 2012
- McCarthy HD, Ashwell M. A study of central fatness using waist-to-height ratios in UK children and adolescents. Int J Obes, 2006