What Is BMI?
Body Mass Index (BMI) is a simple screening tool that estimates body fat based on height and weight. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and has been used in public health research ever since. The formula is: BMI = weight (kg) ÷ height (m)²
BMI Categories (WHO Standards)
- Under 18.5: Underweight — may indicate malnutrition or an underlying health issue
- 18.5–24.9: Normal weight — associated with lowest health risk for most people
- 25.0–29.9: Overweight — moderately elevated health risk
- 30.0–34.9: Obese Class I
- 35.0–39.9: Obese Class II
- 40+: Obese Class III (severe)
Important Limitations of BMI
BMI is a population-level tool with significant individual limitations:
- Muscle mass: Athletes with high muscle mass often have "overweight" or "obese" BMI despite very low body fat
- Bone density: Dense bones increase weight without increasing health risk
- Age: Older adults may have healthy BMI but higher body fat percentage
- Ethnicity: Asian populations have higher health risks at lower BMI values; WHO recommends lower thresholds (23 for overweight, 27.5 for obese)
- Body distribution: BMI doesn't account for where fat is stored; abdominal fat carries much higher risk than hip/thigh fat
Use BMI as one data point alongside waist circumference, body fat percentage, blood markers, and professional medical assessment for a complete health picture.