About

Drug dosing in obese patients requires more than actual scale weight. Standard dosing based on total body weight overestimates lean tissue contribution, risking toxicity with hydrophilic drugs like aminoglycosides and vancomycin. The adjusted body weight (AjBW) corrects for this by blending ideal body weight (IBW) with a fraction of excess adipose mass. The Devine formula (1974) anchors IBW to height in inches above 60 in (152.4 cm). This tool implements the standard correction factor CF = 0.4, adjustable from 0.25 to 0.5 per institutional protocol. Results are approximations assuming normally distributed lean mass. They do not replace clinical judgment for amputees, edematous patients, or pediatric populations.

Formulas

The ideal body weight is calculated using the Devine (1974) formula:

IBW_male = 50 + 2.3 × (h_in − 60)

IBW_female = 45.5 + 2.3 × (h_in − 60)

The adjusted body weight incorporates a correction factor applied to the excess weight above IBW:

AjBW = IBW + CF × (ABW − IBW)

Where IBW = ideal body weight (kg), ABW = actual body weight (kg), CF = correction factor (typically 0.4), h_in = patient height in inches, and AjBW = adjusted body weight (kg). BMI is computed for reference:

BMI = ABWh_m²

Where h_m = height in meters.

Reference Data

Parameter	Male	Female	Notes
IBW Base (at 5 ft / 60 in)	50 kg	45.5 kg	Devine 1974
IBW Increment per inch > 60	2.3 kg/in	2.3 kg/in	Same for both sexes
Standard Correction Factor	0.40		Most common clinical default
CF for Aminoglycosides	0.40		Bauer 1983
CF for Vancomycin	0.40		Murphy 2006
CF for Fluoroquinolones	0.45		Pai 2014
CF for Enoxaparin (DVT prophylaxis)	0.40		Barras 2009
Minimum Height for Devine	60 in (152.4 cm)		Below this, IBW = base value
BMI Threshold: Overweight	25.0 kg/m²		WHO Classification
BMI Threshold: Obese Class I	30.0 kg/m²		AjBW typically warranted
BMI Threshold: Obese Class II	35.0 kg/m²		Severe obesity
BMI Threshold: Obese Class III	40.0 kg/m²		Morbid obesity
ABW < IBW Guidance	Use ABW		AjBW not applicable
ABW = IBW Guidance	Use ABW		No excess weight to adjust
ABW > 120% IBW	Use AjBW		Common clinical cutoff
Lean Body Weight (Boer, Male)	0.407W + 0.267H − 19.2	-	Alternative to AjBW
Lean Body Weight (Boer, Female)	-	0.252W + 0.473H − 48.3	Alternative to AjBW

Frequently Asked Questions

Adjusted body weight is typically used when a patient's actual body weight exceeds 120% of their ideal body weight. This threshold indicates significant excess adipose tissue that does not proportionally increase the volume of distribution for hydrophilic drugs. Drugs commonly dosed on AjBW include aminoglycosides, vancomycin, and heparin. If ABW is below or equal to IBW, use actual body weight directly.

The correction factor of 0.4 originates from pharmacokinetic studies (Bauer 1983) demonstrating that approximately 40% of excess body weight in obese patients contributes to the effective volume of distribution for aminoglycosides. Different drugs distribute differently into adipose tissue. Fluoroquinolones may warrant 0.45. Institutional protocols vary. This calculator allows adjusting CF from 0.25 to 0.50.

The Devine formula uses 60 inches as its baseline. For heights at or below this threshold, the increment term becomes zero or negative. Clinically, the IBW base value (50 kg male, 45.5 kg female) is used as a floor. This calculator enforces the base as the minimum IBW to avoid nonsensical negative adjustments.

BMI provides clinical context. A BMI ≥ 30 kg/m² classifies obesity per WHO standards and generally correlates with ABW exceeding 120% of IBW. This calculator displays BMI and its WHO classification alongside AjBW so clinicians can cross-reference. BMI alone does not determine dosing weight. The ABW-to-IBW ratio is the primary trigger.

The Devine formula was derived from insurance actuarial tables of predominantly Caucasian North American adults. It may overestimate IBW for shorter populations and underestimate it for taller ones. It does not account for muscularity, frame size, or ethnicity-specific body composition. Alternative formulas (Robinson, Miller, Hamwi) exist. For patients with amputations, fluid retention, or pregnancy, the Devine IBW should be clinically adjusted before computing AjBW.

Adjusted body weight adds a fraction of excess weight to IBW. Lean body weight (LBW) estimates fat-free mass using regression equations (e.g., Boer formula) that factor in both height and weight. LBW is preferred for drugs like propofol that distribute primarily into lean tissue. AjBW is simpler and more widely used for antibiotic dosing. They are not interchangeable. This tool calculates AjBW specifically.