Metric formula
Use kilograms and meters:
BMI = weight (kg) / height² (m²)
If you have height in centimeters, convert first: m = cm ÷ 100
Imperial (US) formula
Use pounds and inches:
BMI = 703 × weight (lb) / height² (in²)
Convert feet+inches to total inches: in = (ft × 12) + in
Adult BMI categories (commonly used)
- Underweight: < 18.5
- Healthy weight: 18.5–24.9
- Overweight: 25.0–29.9
- Obesity: 30.0+
When BMI can be misleading
- Very muscular people (BMI can read “high” with low body fat)
- Pregnancy
- Children and teens (must use percentiles)
- Some older adults (body composition changes)
Keep it simple
The formula is easy. The mistake is treating BMI as a diagnosis instead of a screening metric.