Adjusted Body Weight Calculator
Use this adjusted body weight calculator to estimate a clinical reference weight used for dosing above ideal weight.
Updated October 22, 2026
Medical Disclaimer: This calculator provides a general estimate based on established research and is for educational purposes only, not a substitute for medical advice. Individual results can vary based on health conditions, so consult a qualified healthcare professional or registered dietitian before making significant changes to your diet, exercise, or health routine. Read our full Medical Disclaimer.
How Adjusted Body Weight Is Calculated
Adjusted body weight is a reference weight that sits between your ideal weight and your actual weight, used mainly for drug dosing in people well above their ideal weight. Neither actual weight nor ideal weight works well as a dosing reference in that case: actual weight overstates how much metabolically active tissue is really there, and ideal weight understates it. Adjusted body weight splits the difference with a correction factor based on drug levels measured in patients¹.
What This Calculator Measures
This estimates a reference weight used in clinical and pharmacokinetic contexts, most commonly for aminoglycoside antibiotics, such as gentamicin and tobramycin, in patients well above their ideal body weight. It is not a personal health target or body composition measurement.
The Formula
Ideal body weight (Devine, 1974): Men = 50 kg + 2.3 kg per inch over 5 ft. Women = 45.5 kg + 2.3 kg per inch over 5 ft. This calculator uses the Devine formula, which was first published in a paper on gentamicin dosing² and became the ideal weight formula used in drug dosing³.
Adjusted body weight = IBW + 0.4 × (Actual Weight − IBW)
The 0.4 factor counts each kilogram above ideal weight as 0.4 kilogram, because extra weight adds less to the space the drug spreads into (its volume of distribution) than lean weight does. A study of drug levels in 1,708 patients on gentamicin or tobramycin found a factor of 0.43 for overweight patients¹.
How This Calculator Computes Your Result
Enter your sex, height, and actual weight. We calculate ideal body weight using the Devine formula, then apply the 0.4 correction factor to the difference between actual and ideal weight. When actual weight is below or only modestly above ideal weight, roughly under 120% of IBW, the adjustment isn't typically applied, actual or ideal weight is used directly instead.
Why 0.4 Specifically, and Why It's Less Fixed Than It Looks
The 0.4 figure isn't an arbitrary round number. Early work in morbidly obese patients recommended factors of 38% to 58%⁴, and a later study of aminoglycoside levels in 1,708 patients of varying body sizes found that 0.43 best predicted drug distribution in overweight patients¹.
Worth knowing, though: the factor that works best varies with the drug and the patient group. 0.4 is the most commonly used and cited figure, not a universally precise constant.
A separate study of the standard 0.4 factor in 31 morbidly obese patients receiving once-daily aminoglycosides found it produced an appropriate drug level in 71% of cases, with 13% too high and 16% too low⁴. That is useful context for how much precision to expect from this estimate in practice.
A Note on Accuracy
Adjusted body weight is a correction factor built for clinical dosing contexts, not a body composition measurement. It only applies once actual weight is meaningfully above ideal weight, pairing this with the Ideal Weight or BMI calculator gives fuller context. The 0.4 correction factor itself varies meaningfully across studies and drugs, treat it as the standard reference point, not a precisely fixed constant.
Frequently Asked Questions
What is adjusted body weight used for?
It's a reference weight used in clinical and pharmacokinetic contexts, such as medication dosing, for people whose actual weight is well above their ideal weight. It is not a personal health target.
Why not just use actual or ideal weight?
Neither reflects metabolically active tissue well once someone is well above their ideal weight, actual weight overstates it, ideal weight understates it. The 0.4 correction factor splits the difference.
When does this calculator say an adjustment isn't needed?
When your actual weight is under 120% of your ideal weight from the Devine formula. In that range the adjustment isn't typically applied, so the calculator adds a note that actual or ideal weight is generally used directly instead. Hospitals differ on the exact cut-off.
Is this a medical dosing calculation?
No. This is an educational reference, not a substitute for a clinician's or pharmacist's own dosing calculation.
Sources
Aminoglycoside Dosing Weight Correction Factors for Patients of Various Body Sizes, Traynor et al., Antimicrobial Agents and Chemotherapy, 1995
Gentamicin Therapy, Devine, B.J., Drug Intelligence and Clinical Pharmacy, 1974
The Origin of the 'Ideal' Body Weight Equations, Pai & Paloucek, Annals of Pharmacotherapy, 2000
Evaluation of Extended Interval Dosing Aminoglycosides in the Morbidly Obese Population, Ross et al., Advances in Pharmacological Sciences, 2013