Ideal Body Weight Formulas for Adults
For drug dosing in clinical practice, use the modified Devine formula: Men = 51.65 kg + 1.85 kg/inch of height > 5 feet; Women = 48.67 kg + 1.65 kg/inch of height > 5 feet. 1
Primary IBW Formulas
Modified Devine Formula (Preferred for Drug Dosing)
The most widely endorsed formula across multiple guideline societies is the modified Devine equation 1, 2:
- Men: IBW (kg) = 51.65 kg + 1.85 kg/inch of height > 5 feet 1, 2
- Women: IBW (kg) = 48.67 kg + 1.65 kg/inch of height > 5 feet 1, 2
This formula is specifically recommended by the European Society of Cardiology for antithrombotic drug dosing and appears in multiple international guidelines 1.
Simplified Height-Based Formula (Alternative)
A simpler approach endorsed by anesthesiology guidelines 1, 3, 4:
This method is particularly useful for rapid bedside calculations during anesthesia and critical care 1, 4.
Hamwi Method (Quick Estimation)
The KDOQI guidelines describe the Hamwi method, though they caution it lacks scientific validation 1:
- Women: 100 lb (45.36 kg) for first 5'0" + 5 lb (2.27 kg) for each additional inch 1
- Men: 106 lb (48.08 kg) for first 5'0" + 6 lb (2.72 kg) for each additional inch 1
Clinical Application by Context
For Drug Dosing in Obese Patients (BMI ≥30)
Do not use total body weight for most medications—this significantly increases overdose risk, hypotension, and anesthetic awareness. 1, 4
Instead, calculate adjusted body weight (ABW) using 1, 3, 5:
The 0.4 adjustment factor is the standard for anesthetic and most medication dosing 3, 5. This accounts for the fact that approximately 40% of excess adipose tissue is metabolically active 1, 5.
When to use ABW: Apply this formula when actual weight is <95% or >115% of IBW 3.
Special Dosing Adjustments by Drug Class
Different adjustment factors may be appropriate for specific medications 3:
- Nutritional calculations: Use 0.33 adjustment factor (ESPEN recommendation) 3, 5
- Dialysis patients: Use 0.25 adjustment factor (KDOQI recommendation) 3
- Hydrophilic antibiotics: β-lactams (0.3), aminoglycosides (0.4), quinolones (0.45) 3
For Dialysis Patients
When calculating IBW or ABW in dialysis populations 3:
- Hemodialysis: Use post-dialysis (edema-free) weight as the actual weight 3
- Peritoneal dialysis: Use post-drainage weight (after dialysate removal) 3
Important Caveats and Pitfalls
Limitations of IBW Formulas
The KDOQI guidelines emphasize several critical limitations 1:
- IBW is based on Metropolitan Life Insurance data and is not generalizable to CKD populations 1
- The Hamwi method, while convenient, has no scientific validation 1
- IBW formulas tend to overestimate at shorter heights and underestimate at taller heights 6, 7
When IBW May Not Apply
For patients with actual weight between 95-115% of IBW, you can use total body weight directly for dosing and nutritional calculations 3.
Hydrophilic vs. Lipophilic Drugs
Critical distinction: Hydrophilic drugs (like low-molecular-weight heparin) have non-linear volume of distribution increases with body weight, meaning weight-adjusted dosing can cause overdosing in severe obesity 1. Lipophilic drugs have larger volumes of distribution in obese patients but changes are drug-specific 1.
Anesthesia Safety Concern
The NAP5 audit found a disproportionate number of obese patients experienced accidental awareness under general anesthesia, with half occurring during induction when total body weight dosing was used instead of lean or adjusted body weight 1, 4.