What is body mass index (BMI)?
Body mass index (BMI) is a number calculated from weight and height that screens for weight categories linked to health risk. Developed in the 1830s, it remains the most widely used weight assessment tool in clinical practice worldwide because it requires only two measurements.
How is BMI calculated?
BMI = weight (kg) / height (m)²
For example, a person weighing 75 kg and standing 175 cm (1.75 m) tall: BMI = 75 / (1.75 × 1.75) = 24.5 — normal weight. In US customary units: BMI = (weight in lbs × 703) / (height in inches)².
What are the BMI categories according to WHO?
The World Health Organization defines four BMI categories for adults:
- Underweight: BMI below 18.5
- Normal weight: BMI 18.5–24.9
- Overweight: BMI 25–29.9
- Obese: BMI 30 or above (Class I: 30–34.9, Class II: 35–39.9, Class III: 40+)
What health risks are linked to a high or low BMI?
A BMI of 25 or above is associated with increased risk of type 2 diabetes, high blood pressure, coronary heart disease, certain cancers (colon, breast, kidney), and sleep apnea. Risk rises substantially at BMI 30 and above.
A BMI below 18.5 carries its own health risks: malnutrition, osteoporosis, weakened immune function, and anemia. Even within the normal range, excess abdominal fat can elevate cardiovascular risk regardless of overall BMI.
What are some BMI calculation examples?
Woman: 60 kg, 165 cm → BMI = 60 / (1.65 × 1.65) = 22.0 — normal weight.
Man: 95 kg, 180 cm → BMI = 95 / (1.80 × 1.80) = 29.3 — overweight.
Child or teenager: For those under 20, BMI is interpreted using age- and sex-specific percentile charts rather than fixed cut-offs.
What is a normal BMI for adults?
A normal BMI for adults is 18.5–24.9, associated with the lowest risk of chronic disease. The optimal range may shift for older adults — research suggests a BMI of 22–27 may reduce fracture and muscle-loss risk in those over 65.
What are the limitations of BMI?
BMI cannot distinguish fat from muscle — a muscular athlete may be classified as overweight despite very low body fat. It also ignores fat distribution: abdominal fat is far more dangerous than fat stored in the hips and thighs.
Ethnic differences: WHO recommends lower cut-offs for Asian populations — overweight begins at BMI 23 and obesity at 27.5, compared to 25 and 30 for other groups. BMI is also unreliable during pregnancy.
For a fuller picture, the waist-to-height ratio (keep waist circumference below half your height) is increasingly recognized as a stronger predictor of cardiovascular risk than BMI alone.