Adjusted Body Weight Calculator
Calculate adjusted body weight (ABW) for drug dosing in obese patients.
Used in anesthesia and pharmacology for accurate medication calculations.
BMI risk assessment for anesthesia evaluates how body mass index affects anesthetic risk, airway management difficulty, drug dosing, and post-operative complications. Obesity significantly increases anesthetic risk across multiple dimensions — it is one of the most important patient factors an anesthesiologist considers during pre-operative assessment.
BMI formula: BMI = Weight (kg) ÷ Height (m)² Or: BMI = (Weight (lbs) ÷ Height (inches)²) × 703
Anesthesia risk by BMI category:
- BMI < 18.5 (Underweight): Increased risk of respiratory depression, drug sensitivity, poor wound healing
- BMI 18.5–24.9 (Normal): Baseline anesthetic risk
- BMI 25–29.9 (Overweight): Slightly elevated risk; monitoring required
- BMI 30–34.9 (Obese Class I): Elevated risk — airway assessment mandatory; OSA screening required
- BMI 35–39.9 (Obese Class II): Significantly elevated — may require specialized equipment; regional anesthesia preferred
- BMI ≥ 40 (Obese Class III / Morbidly Obese): High risk — prone to difficult intubation, reduced lung capacity, increased DVT risk
Specific risks at BMI ≥ 30:
- Airway: Fatty tissue narrows pharynx; Mallampati score typically higher; higher failed intubation rate
- Respiratory: Reduced FRC (functional residual capacity); desaturation occurs faster during apnea; OSA common
- Cardiovascular: Increased oxygen demand; higher prevalence of hypertension and cardiomyopathy
- Drug dosing: Must use ideal body weight (IBW) for some drugs, actual body weight for others
Ideal Body Weight (IBW) formulas (used for anesthetic drug dosing):
- Male IBW (kg) = 50 + 2.3 × (Height in inches − 60)
- Female IBW (kg) = 45.5 + 2.3 × (Height in inches − 60)
Worked example: Patient: Female, 5'4" (64 inches), 220 lbs (100 kg). Scheduled for elective abdominal surgery.
- BMI = (220 ÷ 64²) × 703 = (220 ÷ 4,096) × 703 = 37.7 (Obese Class II)
- IBW = 45.5 + 2.3 × (64 − 60) = 45.5 + 9.2 = 54.7 kg
- Risk category: Significantly elevated — anesthesiologist will plan for potential difficult airway, have video laryngoscope available, and consider regional techniques to minimize general anesthetic exposure.