BMI Calculation
Body Mass Index (BMI) is calculated as weight in kilograms divided by height in meters squared (kg/m²). 1
The Formula
BMI = weight (kg) ÷ height (m)²
This simple mathematical formula provides a standardized method to assess body weight relative to height across populations 2. The calculation requires:
- Weight measured in kilograms
- Height measured in meters, which must be squared (multiplied by itself)
For example, a person weighing 70 kg with a height of 1.75 m would have a BMI of: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.06 = 22.9 kg/m²
Standard BMI Categories for Adults
The World Health Organization and major medical societies use these classifications 1, 2:
- Underweight: BMI < 18.5 kg/m²
- Normal weight: BMI 18.5–24.9 kg/m²
- Overweight: BMI 25.0–29.9 kg/m²
- Obesity Class I: BMI 30.0–34.9 kg/m²
- Obesity Class II: BMI 35.0–39.9 kg/m²
- Obesity Class III: BMI ≥ 40 kg/m² (severe obesity)
Important Caveats About BMI
BMI does not directly measure body fat or account for individual differences in body composition 1. The calculation uses total body weight without distinguishing between lean mass and fat mass 2. This creates several limitations:
- Athletes and muscular individuals may be misclassified as overweight or obese despite having low body fat
- Individuals with normal weight but excess body fat may not be identified as having obesity
- BMI has only 50% sensitivity for detecting excess adiposity, though it maintains 90% specificity 2
Population-Specific Considerations
Asian populations develop cardiometabolic diseases at lower BMI levels 1. Some expert guidelines recommend lower BMI thresholds for obesity in Asian populations (≥25 or ≥27.5 kg/m²) rather than the standard ≥30 kg/m² cutoff 1, 2.
Additional measurements should complement BMI, particularly waist circumference (≥102 cm for men, ≥88 cm for women), which better reflects visceral adiposity and cardiometabolic risk in patients with BMI 25–34.9 kg/m² 1.
Why Height is Squared (Not Cubed)
The formula uses height² rather than height³ because BMI was designed to be statistically uncorrelated with height in adult populations 3. While mass theoretically varies more closely with height³ in geometric terms, the exponent of 2 provides better statistical independence from height across diverse populations, making BMI a more practical screening tool 3.