Body mass index
Weight-for-height index
- WHO classification
Sources last checked: 2026-10-13
Body mass index
Weight-for-height index
Does not distinguish fat from muscle mass; cut-offs differ for some Asian populations.
Interpretation
| Underweight | < 18.5 kg/m² |
|---|---|
| Normal range | 18.5–24.9 kg/m² |
| Overweight (pre-obese) | 25.0–29.9 kg/m² |
| Obesity class I | 30.0–34.9 kg/m² |
| Obesity class II | 35.0–39.9 kg/m² |
| Obesity class III | ≥ 40 kg/m² |
What is the Body mass index?
Body mass index is weight in kilograms divided by height in metres squared. The WHO classification (Technical Report Series 894, 2000) defines underweight, normal weight, overweight and three classes of obesity for adults. It is a population screening measure, not a direct measure of body fat.
When to use it
- Screening adults for underweight, overweight and obesity in primary care and hospital admission.
- Eligibility thresholds for obesity pharmacotherapy and bariatric surgery referral.
- Nutritional assessment and as one input to drug dosing or anaesthesia planning.
When not to use it
- Children and adolescents — use age- and sex-specific BMI percentiles.
- Pregnancy, marked oedema or ascites, and very muscular individuals, where weight does not reflect adiposity.
- As the sole indicator of metabolic risk — waist circumference adds information.
How it is calculated
BMI = weight (kg) / height (m)²
Clinical pearls and limitations
- Does not distinguish fat from muscle mass; cut-offs differ for some Asian populations.
- WHO adult classes: < 18.5 underweight, 18.5–24.9 normal, 25.0–29.9 pre-obese, 30.0–34.9 obesity I, 35.0–39.9 obesity II, ≥ 40 obesity III.
- Lower action points are recommended for people of Asian descent because of higher cardiometabolic risk at a given BMI.
- In older adults, a BMI slightly above 25 is not associated with excess mortality; unintentional weight loss matters more.
- Measure height rather than relying on self-report — small height errors produce large BMI errors.
Frequently asked questions
What BMI counts as obesity?
In adults, WHO defines obesity as BMI ≥ 30 kg/m², subdivided into class I (30–34.9), class II (35–39.9) and class III (≥ 40).
Is BMI accurate for athletes?
No. BMI cannot distinguish muscle from fat, so muscular people can be classed as overweight with normal body fat. Waist circumference or body composition measurement is more informative.
How do I calculate BMI by hand?
Divide weight in kg by height in metres squared: 70 kg / (1.75 m × 1.75 m) = 22.9 kg/m².
Evidence and guidelines
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