Adjusted Body Weight Calculator
Adjusted Body Weight Calculator
Adjusted body weight is ideal body weight plus 40% of the excess — the dosing weight for aminoglycosides in obesity. This works it out, says whether the usual trigger (actual weight more than 20% above ideal) is met, and declines to adjust when actual weight is below ideal.
Adjusted body weight
IBW + 0.4 × excessMan, 178 cm, actual weight 110 kg
Adjusted body weight
- IBW
- Devine ideal body weight from sex and height
- 0.4
- the fraction of excess weight into which aminoglycosides distribute, from Bauer 1983
- trigger
- actual weight more than 120% of IBW (Nicolau 1995); other protocols use 125%, 130% or BMI 40
- below IBW
- no adjustment; use actual weight
Worked example
Man, 178 cm, actual weight 110 kg
IBW = 50 + 2.3 × (70.08 − 60) = 73.18 kg
110 ÷ 73.18 = 150% of ideal — more than 120%, so the adjustment applies
Excess = 110 − 73.18 = 36.82 kg
0.4 × 36.82 = 14.73 kg
73.18 + 14.73 = 87.9 kg
Which weight the Hartford program dosed on
| Actual weight | Dosing weight | Source |
|---|---|---|
| Below ideal | Actual weight | Nicolau 1995: actual weight unless obese |
| Ideal to 120% of ideal | Actual weight | Nicolau 1995 |
| More than 120% of ideal | IBW + 0.4 × (actual − IBW) | Nicolau 1995; 0.4 from Bauer 1983 |
The trigger varies between protocols
| Trigger for adjusting | Where it is used |
|---|---|
| Actual more than 120% of IBW | Hartford once-daily aminoglycoside program (Nicolau 1995) |
| Actual more than 130% of IBW | Some Cockcroft-Gault conventions |
| BMI 40 or more | Some critical care dosing references |
| "Obese", undefined | Several institutional protocols |
Why 40%, and why only above 120%
Adjusted body weight exists because neither of the obvious weights works for an aminoglycoside in an obese patient. Gentamicin and tobramycin distribute into extracellular water, which lean tissue has plenty of and fat has little of — but not none. Dose on total weight and the peak overshoots; dose on ideal weight, which ignores the extra tissue entirely, and it undershoots. Bauer and colleagues measured aminoglycoside volume of distribution in morbidly obese patients in 1983 and found it behaved as if the drug distributed into ideal weight plus about 40% of the excess. That is the whole origin of the 0.4 in the formula.
The Hartford once-daily aminoglycoside program (Nicolau 1995), which gave 7 mg/kg to 2,184 adults, used this weight only when the patient was obese, defined as more than 20% over ideal body weight. Below that it dosed on actual weight — including in patients whose actual weight was below ideal. This page follows that convention: it shows the adjusted figure, says whether the trigger is met, and returns actual weight rather than a nonsense number when actual is below ideal.
Two cautions. First, 0.4 is an aminoglycoside number. A review for the American College of Clinical Pharmacy reports published correction factors from 0.14 to 0.98 across drugs, so the same arithmetic applied to another drug is a convention, not a measurement. Second, the trigger varies: 120%, 125%, 130% of ideal and a BMI of 40 are all in use. Adjusted weight is also widely put into Cockcroft-Gault in obesity, where the evidence is more divided — one study in morbid obesity found it overestimated measured clearance by about 30%. The dosing weight selection page sets out those disagreements. The ideal body weight calculator explains where ideal weight comes from, and the lean body weight calculator gives the alternative size descriptor that some of that evidence prefers.
Frequently asked questions
How do you calculate adjusted body weight?
Adjusted body weight = ideal body weight + 0.4 × (actual weight − ideal body weight). For a man of 178 cm (IBW 73.2 kg) weighing 110 kg, the excess is 36.8 kg, 40% of that is 14.7 kg, and the adjusted weight is 87.9 kg.
When should adjusted body weight be used?
The classic use is aminoglycoside dosing in obesity. The Hartford once-daily program (Nicolau 1995) used it when actual weight was more than 20% over ideal body weight, and used actual weight otherwise. Other protocols trigger at 125% or 130% of ideal, or at a BMI of 40, and many also use it in Cockcroft-Gault for obese patients. Use the trigger your protocol specifies.
Where does the 0.4 factor come from?
From Bauer and colleagues (1983), who studied aminoglycoside pharmacokinetics in normal-weight and morbidly obese patients and found the volume of distribution matched ideal weight plus about 40% of the excess. It describes aminoglycosides. Correction factors reported for other drugs range from 0.14 to 0.98, so 0.4 should not be assumed to apply to them.
What if the patient weighs less than their ideal body weight?
Do not adjust. With actual weight below ideal, the formula returns a weight heavier than the patient, which no convention intends. The Hartford program dosed on actual weight in every patient who was not obese, and this page returns the actual weight in that case.
Is adjusted body weight the same as lean body weight?
No. Adjusted weight is ideal weight plus a fixed fraction of the excess — a convention built for aminoglycosides. Lean body weight (Janmahasatian 2005) is estimated from weight and BMI and rises with actual weight towards a ceiling; it is a size descriptor rather than a drug-specific weight. They can differ by 15 kg or more in the same patient.
Related calculators
References
- Bauer LA, Edwards WA, Dellinger EP, Simonowitz DA. Influence of weight on aminoglycoside pharmacokinetics in normal weight and morbidly obese patients. Eur J Clin Pharmacol. 1983;24(5):643–647. The origin of the 0.4 correction factor in adjusted body weight.
- Nicolau DP, Freeman CD, Belliveau PP, Nightingale CH, Ross JW, Quintiliani R. Experience with a once-daily aminoglycoside program administered to 2,184 adult patients. Antimicrob Agents Chemother. 1995;39(3):650–655. Fixed 7 mg/kg dose; actual body weight unless the patient was obese ("20% over ideal body weight"), when the dose-determining weight was ideal body weight + 0.4 × (actual − ideal); a single level 6–14 hours after the start of the infusion read against the Hartford nomogram; ascites, burns over 20% of body surface area, pregnancy, end-stage renal disease and enterococcal endocarditis excluded, as were children.
- Barletta JF. Drug dosing in special populations: obesity and geriatrics. In: CCSAP 2020 Book 2, Issues in Critical Care Practice. Lenexa, KS: American College of Clinical Pharmacy; 2020. Transcribes the Janmahasatian and Devine equations; names Bauer 1983 as the source of the 0.4 factor and reports published correction factors from 0.14 to 0.98; summarises Demirovic 2009 and the heparin literature.
- Devine BJ. Gentamicin therapy. Drug Intell Clin Pharm. 1974;8:650–655. The source of the Devine equation: men 50 kg + 2.3 kg per inch over 5 feet, women 45.5 kg + 2.3 kg per inch over 5 feet, proposed for estimating gentamicin doses.
- Demirovic JA, Pai AB, Pai MP. Estimation of creatinine clearance in morbidly obese patients. Am J Health Syst Pharm. 2009;66(7):642–648. Against measured clearance, total body weight overestimated by about two-fold, ideal body weight underestimated by about 22%, the 0.4-adjusted weight overestimated by about 30%, and lean body weight was the most accurate.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.