BMI Formula
BMI is calculated as body weight in kilograms divided by height in meters squared (kg/m²). 1
Standard Calculation Methods
The formula can be expressed in three equivalent ways depending on your measurement units:
- Metric formula: BMI = weight (kg) / [height (m)]² 1
- Alternative metric: BMI = weight (kg) / height (cm) / height (cm) × 10,000 1
- Imperial formula: BMI = weight (lb) / height (in) / height (in) × 703 1
Calculation Example
For practical application, if measuring a patient who is 157.48 cm tall (1.5748 m) and weighs 68 kg 2:
Clinical Interpretation
Standard BMI categories for most populations 1, 2:
- Normal/Healthy weight: 18.5-24.9 kg/m² 2
- Overweight: 25.0-29.9 kg/m² 1, 2
- Class I obesity: 30.0-34.9 kg/m² 1, 2
- Class II obesity: 35.0-39.9 kg/m² 1, 2
- Class III obesity: ≥40.0 kg/m² 1, 2
Population-Specific Adjustments
For Asian and Asian American populations, use lower BMI thresholds due to higher body fat percentages and visceral adiposity at equivalent BMI values 1, 2:
These lower cutoffs reflect that Asian populations have 2-3 kg/m² lower BMIs than Caucasians for the same body fat percentage and higher insulin resistance even at lower BMI values 2.
Critical Limitations to Recognize
BMI does not distinguish between lean and fat mass, leading to potential misclassification 1:
- Individuals with normal BMI but high body fat (≥23% in men, ≥33% in women) have 4-7 times higher risk of metabolic syndrome and increased mortality 1
- Athletes or highly muscular individuals may be incorrectly classified as overweight or obese 1
- BMI has only 36% sensitivity for detecting excess body fat in men and 49% in women, though specificity is high (95% in men, 99% in women) 1
Always measure waist circumference alongside BMI for comprehensive risk assessment 1, 2: