How is ideal body weight calculated?

The Devine formula: Males: IBW (kg) = 50 + 2.3 × (height in inches − 60). Females: IBW (kg) = 45.5 + 2.3 × (height in inches − 60). For patients shorter than 5 feet, alternative formulas may be more appropriate.

When is ideal body weight used clinically?

IBW is used for lung-protective ventilation (tidal volume 6–8 mL/kg IBW), aminoglycoside dosing (loading dose), some anesthetic dosing, and as a reference for nutritional assessment. It is also used to calculate adjusted body weight for drug dosing in obese patients.

What is the difference between IBW and adjusted body weight?

Ideal body weight (IBW) is based solely on height and sex. Adjusted body weight (AdjBW) = IBW + 0.4 × (actual weight − IBW) accounts for the increased volume of distribution in obese patients. AdjBW is used for vancomycin dosing and some other drug calculations in obesity.

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Ideal Body Weight Calculator

Calculate IBW (Devine formula) and Adjusted Body Weight (AdjBW) for drug dosing calculations.

Inputs

Sex

Height (cm) Must be > 152.4 cm (5 ft) for Devine formula

Actual Weight (kg) (optional, for AdjBW)

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Enter sex and height to calculate IBW.

IBW (♂)50 + 2.3 × (height in inches − 60)

IBW (♀)45.5 + 2.3 × (height in inches − 60)

AdjBWIBW + 0.4 × (Actual − IBW)

Disclaimer: For educational purposes only. Not a substitute for clinical judgment.

When to use

IBW is commonly used for tidal volume calculation in mechanical ventilation (6–8 mL/kg IBW) and for weight-based drug dosing in obese patients (e.g., aminoglycosides, vancomycin, enoxaparin).

Clinical pearls

Use IBW for lung-protective ventilation tidal volumes — actual body weight causes volutrauma in obese patients.
Adjusted Body Weight (AdjBW) with a 0.4 correction factor is used for aminoglycoside and vancomycin dosing in obese patients.
For patients shorter than 5 feet (152.4 cm), IBW should be calculated as the actual weight or use alternative methods.
Different drugs use different weight metrics — always check dosing guidelines for each specific medication.

References

Devine BJ. Gentamicin therapy. Drug Intell Clin Pharm. 1974;8:650-655.
ARDSNet. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury. N Engl J Med. 2000;342(18):1301-1308.

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About This Tool

What Is Ideal Body Weight?

Ideal Body Weight (IBW) is an estimate of what a person should weigh based on their height and sex, independent of their actual body composition. The most widely used formula is the Devine formula (1974), originally developed for drug dosing calculations: for males, IBW (kg) = 50 + 2.3 × (height in inches − 60); for females, IBW (kg) = 45.5 + 2.3 × (height in inches − 60). Despite its simplicity and limitations, this formula remains deeply embedded in clinical practice, particularly for ventilator management and pharmacokinetic dosing.

Adjusted Body Weight for Dosing

In obese patients (typically defined as > 130% of IBW), many drugs require dosing adjustments because neither IBW nor actual body weight (ABW) accurately reflects the relevant volume of distribution. Adjusted Body Weight (AdjBW) uses a correction factor: AdjBW = IBW + 0.4 × (ABW − IBW). The 0.4 factor accounts for the fact that adipose tissue has reduced but not negligible blood flow and drug distribution. AdjBW is commonly used for aminoglycoside dosing, vancomycin initial dosing, and some chemotherapy calculations.

Clinical Applications

The most critical application of IBW is in lung-protective ventilation, where the ARDSNet protocol mandates tidal volumes of 6–8 mL/kg of predicted body weight (essentially IBW). Using actual body weight in obese patients leads to excessive tidal volumes and ventilator-induced lung injury (VILI), since lung size correlates with height, not weight. IBW is also used in Cockcroft-Gault creatinine clearance estimation when patients are at or below IBW.

🔑 Clinical Pearls

ALWAYS use IBW (not actual weight) for ventilator tidal volume settings — lung size scales with height, not weight.
For aminoglycosides in obesity: dose based on AdjBW with 0.4 factor; monitor levels closely.
The Devine formula was never validated — it was created for a drug dosing paper and became standard through convention.
For patients shorter than 5 ft, use actual weight if less than calculated IBW; the formula produces unreliable results below this height.

Key References

Devine BJ. Gentamicin therapy. Drug Intell Clin Pharm. 1974;8:650-655.
The ARDS Network. Ventilation with lower tidal volumes for ALI and ARDS. N Engl J Med. 2000;342(18):1301-1308.
Pai MP, Paloucek FP. The origin of the "ideal" body weight equations. Ann Pharmacother. 2000;34(9):1066-1069.

Formula last verified: February 2026