Almost every BMI calculator you’ll find, including ours, uses the World Health Organization’s adult categories: under 18.5 is underweight, 18.5 to 24.9 is a healthy weight, 25 to 29.9 is overweight and 30 or more is obesity. But the full WHO system has more detail than that four-line summary. There are three separate obesity classes, lower “action points” for Asian populations, and a completely different method for children and teenagers.
Here’s the whole picture in one place.
WHO BMI Categories for Adults (18 and Over)
| BMI (kg/m²) | WHO classification |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Normal (healthy) weight |
| 25.0 to 29.9 | Overweight (pre-obesity) |
| 30.0 to 34.9 | Obesity class I |
| 35.0 to 39.9 | Obesity class II |
| 40.0 and above | Obesity class III |
These are the same for men and women and for all adult ages. BMI itself is simply your weight in kilograms divided by your height in metres squared. If you’d like to do it by hand, our step-by-step guide shows the maths on any phone calculator.
Lower BMI Action Points for Asian Populations
A 2004 WHO expert consultation, published in The Lancet, found that many Asian populations face a higher risk of type 2 diabetes and heart disease at BMIs below the standard overweight cut-off of 25. Rather than replace the international categories, it recommended extra “public health action points”:
- 23: increased risk begins for many Asian populations
- 27.5: high risk
- Further action points at 32.5 and 37.5
In practice, several countries in South and East Asia use 23 as the overweight threshold and 25 or 27.5 for obesity. If you’re of Asian descent and your BMI is between 23 and 25, it’s worth paying closer attention to your waist size (see below), even though a standard calculator will call you “normal”.
How the WHO Measures BMI for Children and Teens (5 to 19)
Children’s bodies change so much as they grow that fixed adult cut-offs don’t work. Instead, the WHO uses the 2007 WHO Growth Reference for 5 to 19 years, which compares a child’s BMI with other children of the same age and sex. The result is expressed as a z-score, the number of standard deviations (SD) above or below the reference median:
| BMI-for-age (WHO, 5 to 19) | Classification |
|---|---|
| Above +2 SD | Obesity (equivalent to BMI 30 at age 19) |
| Above +1 SD | Overweight (equivalent to BMI 25 at age 19) |
| Between −2 SD and +1 SD | Normal |
| Below −2 SD | Thinness |
| Below −3 SD | Severe thinness |
The curves were built so that at age 19 they line up with the adult cut-offs of 25 and 30, giving a smooth hand-over into adult BMI.
WHO vs CDC for Children
In the United States, the CDC uses its own growth charts for ages 2 to 19 and expresses the result as a percentile instead of a z-score:
- Below the 5th percentile: underweight
- 5th to below the 85th: healthy weight
- 85th to below the 95th: overweight
- 95th percentile and above: obesity
Both systems do the same job with slightly different reference data, so a child near a boundary can be classified differently by each. Follow whichever system your child’s doctor uses.
How to Check Your Child’s BMI-for-Age
- Measure height and weight accurately (no shoes, light clothing).
- Calculate BMI the normal way: weight (kg) ÷ height (m)².
- Look up the result on the WHO BMI-for-age chart for your child’s sex and exact age in months, or use the CDC’s official child and teen BMI calculator.
Important: our BMI Calculator uses the adult categories and is designed for people aged 18 and over. For anyone under 18, use the WHO charts or CDC calculator linked below, and talk to your child’s doctor before drawing conclusions. Children’s growth is rarely a straight line.
What the Categories Don’t Tell You
The WHO designed BMI categories as a population screening tool, not an individual diagnosis. Two people with a BMI of 27 can have very different health, depending on how much of their weight is muscle and where their fat is stored. That’s why the WHO, NICE and other bodies also look at waist size: a waist under half your height is a simple, reliable sign of lower risk. See What to Use Instead of BMI for the tape-measure methods doctors increasingly pair with it.
Frequently Asked Questions
What is a normal BMI according to the WHO?
For adults, 18.5 to 24.9 kg/m². Many Asian countries use a lower upper limit of 23, following the WHO’s Asian action points.
What are the WHO obesity classes?
Class I is a BMI of 30 to 34.9, class II is 35 to 39.9, and class III is 40 or above.
Can I use an adult BMI calculator for a child?
You can calculate the BMI number the same way, but you can’t use the adult categories to interpret it. For ages 5 to 19, the WHO compares the number with children of the same age and sex using BMI-for-age charts.
Is the WHO BMI chart the same for men and women?
For adults, yes. The categories are identical. For children and teens, there are separate charts for boys and girls.
Check your adult BMI and WHO category instantly with the free BMI Calculator, which also shows your healthy weight range and a body fat estimate. Building healthier habits? Our free Fitness Planner and printable planners in our Etsy shop can help you stay consistent.
This article is for general information only and is not medical advice. For children and teens especially, talk to a doctor or paediatrician.
Sources: WHO Growth Reference 5–19 years: BMI-for-age; WHO Expert Consultation, The Lancet (2004); CDC Child and Teen BMI Calculator.
[…] For women of South Asian, Chinese and some other Asian backgrounds, health agencies often use lower thresholds because risk of diabetes and heart disease rises at lower BMIs: overweight from 23 and obesity from 27.5. See WHO BMI categories explained. […]