Predicted Body Weight Calculation
For patients with ARDS or COPD requiring mechanical ventilation, use the NIH ARDS Network formula: Males: PBW (kg) = 50 + 2.3 × [height (inches) - 60]; Females: PBW (kg) = 45.5 + 2.3 × [height (inches) - 60]. 1, 2
Standard Formula for Mechanical Ventilation
The ARDSNet predicted body weight equation is the gold standard for calculating tidal volumes in mechanically ventilated patients, particularly those with ARDS or sepsis-induced respiratory failure 3:
- Males: PBW (kg) = 50 + 2.3 × [height (inches) - 60] 4, 1
- Females: PBW (kg) = 45.5 + 2.3 × [height (inches) - 60] 4, 1
This formula should be used because it was validated in the landmark ARDS Network trial that established 6 mL/kg PBW as the target tidal volume, reducing mortality from 39.8% to 31.0%. 1, 3
Why This Formula Matters
Different PBW equations can produce weight differences exceeding 30% in women and 24% in men, particularly in older and shorter patients 3. Using the ARDSNet formula ensures consistency with evidence-based ventilation protocols that have demonstrated mortality benefit 1, 3.
Alternative Formulas for Other Clinical Contexts
Hamwi Method (Quick Estimation)
For general clinical use outside mechanical ventilation 4:
- Women: 100 lb (45.36 kg) for first 5'0" + 5 lb (2.27 kg) for each additional inch 4
- Men: 106 lb (48.08 kg) for first 5'0" + 6 lb (2.72 kg) for each additional inch 4
BMI-Based Formula
For calculating ideal body weight at BMI 22 kg/m² 5, 6:
- IBW (kg) = 22 × height (meters)² 5
Devine Formula (Drug Dosing)
For pharmacokinetic calculations 7:
- Men: IBW (kg) = 51.65 + 1.85 × [height (inches) - 60] 7
- Women: IBW (kg) = 48.67 + 1.65 × [height (inches) - 60] 7
Critical Considerations for ARDS/COPD Patients
Fluid Overload Adjustment
Patients with acute kidney injury or fluid overload require edema-free weight estimation 4. The reference body weight for energy calculations remains controversial, as Harris-Benedict equations using actual, ideal, or predicted body weight all show poor precision compared to indirect calorimetry 4.
Obesity Adjustments
For obese patients requiring adjusted body weight for nutritional calculations 4:
- Adjusted BW (kg) = IBW + [(actual BW - IBW) × 0.25] 4
This assumes 25% of excess adipose tissue is metabolically active, though this has not been validated in CKD or critical illness 4.
Common Pitfalls
Never use actual body weight for tidal volume calculations in mechanically ventilated patients—this leads to ventilator-induced lung injury and increased mortality 4, 1. The target of 6 mL/kg must be based on predicted body weight, not actual weight 1, 2.
Do not interchange PBW formulas across studies or clinical contexts—mathematical differences between equations can reach 24% in women and 15% in men, potentially causing significant under- or over-dosing of interventions 3.
Avoid using predictive equations for energy requirements in critically ill patients with AKI—these formulas have wide limits of agreement and frequently over- or underestimate actual energy expenditure regardless of which body weight is used 4.