Adjusted Body Weight Calculator
Adjusted body weight (ABW) provides a weight estimate for drug dosing and nutrition assessment in obese patients, where using actual body weight risks overdosing and ideal body weight (IBW) alone may underdose. The formula ABW = IBW + 0.4 x (actual weight - IBW) captures about 40% of the excess weight above IBW, reflecting the proportion of adipose tissue that participates in drug distribution. ABW is most commonly applied to aminoglycoside dosing, some pharmacokinetic calculations, and certain nutritional protocols. Enter the patient's IBW (from the Devine formula) and actual weight, both in kilograms.
ABW formula
ABW = IBW + 0.4 x (Actual Weight - IBW)
ABW is only calculated when actual weight exceeds IBW (obesity). If actual weight is at or below IBW, use actual body weight. IBW should be calculated using the Devine formula (male: 50 + 2.3 per inch above 60; female: 45.5 + 2.3 per inch above 60).
When to use each weight
- Actual weight: Use when patient is not obese (actual weight at or below IBW), or for medications where obesity does not significantly alter pharmacokinetics.
- IBW: Use for mechanical ventilator tidal volume (predicted body weight) and for some drug dosing in morbidly obese patients with hydrophilic drugs.
- ABW: Use for aminoglycoside dosing, some renal function estimates (Cockcroft-Gault in obesity), and nutritional calculations in obese patients.
Frequently asked questions
What is adjusted body weight?
Adjusted body weight (ABW) is an intermediate value between ideal body weight (IBW) and actual body weight used for drug dosing in obese patients. The formula ABW = IBW + 0.4 x (actual - IBW) accounts for the fact that obese adipose tissue has some drug-distributing capacity (about 40% of excess weight), particularly for lipophilic drugs.
When is ABW used instead of IBW or actual weight?
ABW is commonly used for aminoglycoside dosing (gentamicin, tobramycin, amikacin) and some other medications in patients who are obese (actual weight more than 30% above IBW, or BMI above 30 kg/m2). Using actual body weight for these drugs in obese patients risks toxicity; IBW alone may underdose.
Why is the correction factor 0.4?
The 0.4 factor (40%) reflects empirical data showing that approximately 40% of excess weight in obese patients contributes to the volume of distribution for drugs like aminoglycosides. This factor is specific to this drug class and was derived from pharmacokinetic studies. Other drugs may use different obesity correction factors.
How do I find ideal body weight to use in this formula?
Use the Devine formula: Male IBW = 50 kg + 2.3 kg per inch above 60 inches. Female IBW = 45.5 kg + 2.3 kg per inch above 60 inches. This calculator accepts IBW directly. You can calculate IBW first using our Ideal Body Weight calculator and paste the result here.
Is ABW used for nutritional assessment?
In clinical nutrition, ABW or IBW is sometimes used to set protein and calorie targets for obese patients, as providing calories based on actual weight risks overfeeding. However, practice varies by institution and the ARDSNet protocol uses predicted body weight based on height and sex rather than ABW.
Official sources
- NIH National Library of Medicine, PubMed: Pharmacokinetic references for aminoglycoside dosing in obesity.
- FDA Drug Safety: FDA Drug Safety and Availability.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.