Weight relative to height. A screening tool, not a diagnosis.
Source: WHO Physical Status report (1995).
What does BMI mean?
Body Mass Index (BMI) is calculated by dividing your weight in kilograms by the square of your height in metres (kg/m²). The formula was developed by Belgian mathematician Adolphe Quetelet in the 19th century and adopted by the World Health Organization as a population-level screening tool.
BMI tells you whether your weight falls within a range that is statistically associated with increased health risk. A higher BMI is linked to greater likelihood of type 2 diabetes, cardiovascular disease, certain cancers, and joint problems. A very low BMI is associated with malnutrition, bone loss, and immune dysfunction.
Because it uses only two measurements, BMI cannot tell fat apart from muscle. A rugby player and an inactive person of the same height and weight get the same BMI despite very different bodies, so it works best as a first check alongside waist circumference and, where you have it, body fat percentage.
Asian populations tend to experience metabolic complications at lower BMI thresholds, which is why some guidelines recommend an "overweight" cut-point of 23 rather than 25 for those groups. The WHO thresholds below apply to most adult populations.
Reference ranges
| Category | BMI Range | Health implications |
|---|---|---|
| Underweight | < 18.5 | Risk of malnutrition, bone loss, anaemia |
| Healthy weight | 18.5 – 24.9 | Lowest all-cause mortality risk |
| Overweight | 25 – 29.9 | Elevated risk of metabolic disease |
| Obese class I | 30 – 34.9 | Moderate risk; lifestyle intervention recommended |
| Obese class II | 35 – 39.9 | High risk; medical management usually indicated |
| Obese class III | ≥ 40 | Very high risk; specialist care often required |
When should you see a doctor?
See a doctor if your BMI is below 18.5 or above 30, if you have lost or gained a lot of weight without trying, or if your BMI looks healthy but you have other risk factors such as high blood pressure, raised blood sugar, or a family history of heart or metabolic disease. BMI alone never tells the full story, and a clinician can run further tests to fill it in.