What is the formula to calculate ideal body weight?

Ideal Body Weight Formulas

The most widely accepted formula for calculating ideal body weight (IBW) is the modified Devine formula: for men, IBW = 50 kg + 2.3 kg for each inch over 5 feet; for women, IBW = 45.5 kg + 2.3 kg for each inch over 5 feet. 1

Standard Formulas for Ideal Body Weight

• The modified Devine formula in metric units is:

  • Men: IBW = 51.65 kg + 1.85 kg per inch over 5 feet
  • Women: IBW = 48.67 kg + 1.65 kg per inch over 5 feet 1

• Alternatively, the formula can be expressed in terms of height in centimeters:

  • Men: IBW = 50 kg + 0.91 × (height in cm - 152.4)
  • Women: IBW = 45.5 kg + 0.91 × (height in cm - 152.4) 1

Modern Universal Equation

• A more recent universal equation that aligns with BMI values and works for both men and women is:

  • Weight = 2.2 × BMI + 3.5 × BMI × (Height - 1.5 m) 2
  • This formula allows calculation of body weight for any target BMI (typically 22-25 kg/m²) 2

• A simpler formula that yields values within the same range is:

  • IBW = 22 × H², where H is height in meters 3

Clinical Applications and Considerations

• IBW calculations are particularly important for medication dosing, especially for drugs with narrow therapeutic windows 1

• For obese patients (BMI > 30 kg/m²), using IBW rather than actual body weight helps prevent overdosing of medications 4

• When administering intravenous lidocaine, ideal body weight should be used for dose calculation, which can be determined using:

  • Men: IBW = (height in cm - 100)
  • Women: IBW = (height in cm - 105) 4

• For patients with normal BMI (18.5-24.9 kg/m²), actual body weight and ideal body weight are similar 4

Important Caveats

• BMI-based IBW calculations do not account for differences in body composition, potentially misclassifying individuals with high muscle mass 4

• BMI has good specificity (90-99%) but poor sensitivity (36-50%) for detecting obesity, meaning many individuals with excess body fat may have a normal BMI 4

• Age should be considered when determining ideal weight, as optimal BMI tends to be higher for older adults compared to younger populations 5

• For clinical purposes, IBW should be considered as a range rather than a single value, typically corresponding to BMI values between 18.5-24.9 kg/m² 2

• For patients with significant obesity, an alternative measure called normalized lean weight (NLW) may be more appropriate: NLW = IBW + 0.4 × (actual weight - IBW) 6

By using these formulas, clinicians can more accurately determine appropriate medication dosages and set realistic weight goals for patients based on their individual characteristics.