Metric: \( \text{BMI} = \frac{\text{kg}}{(\text{m})^2} \). US: \( \text{BMI} = \frac{\text{lb}}{(\text{in})^2} \times 703 \). Adult categories: Underweight <18.5, Healthy 18.5–24.9, Overweight 25–29.9, Obesity ≥30 (with classes I–III at 30–34.9, 35–39.9, ≥40). This is a screening tool, not a diagnosis.