Formulas for Calculating Ideal Body Weight
The most widely accepted formula for calculating ideal body weight (IBW) is the Devine formula: for men, IBW (kg) = 50 + 2.3 × (height in inches - 60); for women, IBW (kg) = 45.5 + 2.3 × (height in inches - 60).
Traditional IBW Formulas
Devine Formula (1983)
- Men: IBW (kg) = 50 + 2.3 × (height in inches - 60)
- Women: IBW (kg) = 45.5 + 2.3 × (height in inches - 60) 1
This formula is widely used in clinical practice and pharmaceutical dosing calculations.
Alternative Formulas
Metric Version of Devine Formula
- Men: IBW (kg) = 51.65 + 1.85 × (height in inches - 60)
- Women: IBW (kg) = 48.67 + 1.65 × (height in inches - 60) 1
Simple BMI-Based Formula (2005)
- Both genders: IBW (kg) = 22 × H², where H is height in meters 2
This formula provides a weight corresponding to a BMI of 22 kg/m², which falls within the normal BMI range.
Modern Universal Approaches
Universal BMI-Based Formula (2016)
This approach aligns IBW calculations with BMI values, allowing clinicians to calculate body weight for any target BMI:
- Body weight at any BMI = (BMI × height²), where height is in meters 3
For standard IBW calculations, use a target BMI of 22-23 kg/m².
Normalized Lean Weight Formula (2020)
For patients with obesity, particularly when calculating medication dosages:
- NLW (kg) = (9270 × total weight) ÷ (6680 + 216 × BMI) for men
- NLW (kg) = (9270 × total weight) ÷ (8780 + 244 × BMI) for women 4
Adjusted Body Weight
For obese patients, adjusted body weight is often used for certain medication dosing:
- ABW (kg) = IBW (kg) + 0.4 × (total weight - IBW) 5
Special Considerations
Frame Size Adjustments
Frame size can be calculated using the frame index:
- Frame Index = [Elbow Breadth (mm)/Height (cm)] × 100 6
For more precise IBW calculations, refer to frame-specific tables when available.
Amputations
When calculating IBW for patients with amputations, adjust for the percentage of body weight lost:
- Foot: 1.8%
- Leg below knee: 6.5%
- Leg above knee: 8.0%
- Leg at hip: 18.5%
- Hand: 0.8%
- Arm at elbow: 3.1%
- Arm at shoulder: 6.6% 6
Clinical Application
When using IBW for clinical purposes:
- Select the appropriate formula based on the clinical context
- For medication dosing, consider using IBW for normal-weight patients and adjusted body weight for obese patients
- For nutritional assessment, IBW helps determine appropriate caloric and protein requirements
- For renal patients, IBW is particularly important for normalizing protein nitrogen appearance (PNA) 6
Common Pitfalls
- Using actual body weight instead of IBW for patients with obesity or edema
- Failing to account for frame size in IBW calculations
- Not adjusting for amputations when applicable
- Using IBW formulas that don't align with BMI standards, leading to overestimation for tall individuals and underestimation for short individuals 3
Remember that IBW calculations are estimates and should be considered alongside clinical judgment when making treatment decisions.