Dosing Weight Calculator: Actual, Ideal, Adjusted or Lean?
Dosing Weight Calculator: Actual, Ideal, Adjusted or Lean?
Which body weight to dose on — actual, ideal, adjusted or lean — for aminoglycosides, vancomycin, Cockcroft-Gault creatinine clearance and unfractionated heparin, with the number and the source for each. Where published guidance disagrees, as it does for Cockcroft-Gault and heparin in obesity, the page prints the disagreement instead of choosing silently.
Which dosing weight
Drug + patient → which weightCockcroft-Gault, man, 178 cm, 110 kg
How the weight is chosen
Vancomycin loading: actual weight
Cockcroft-Gault: actual weight; if BMI 30 or more, conventions disagree (adjusted shown)
Heparin: actual weight; in obesity adjusted weight is suggested by later studies
- IBW
- Devine: 50 kg (men) or 45.5 kg (women) + 2.3 kg per inch over 5 feet
- adjusted
- IBW + 0.4 × (actual − IBW); never used when actual is below IBW
- lean
- Janmahasatian 2005: 9270 × weight ÷ (6680 + 216 × BMI) in men, (8780 + 244 × BMI) in women
Worked example
Cockcroft-Gault, man, 178 cm, 110 kg
IBW = 73.18 kg; BMI = 110 ÷ 1.78² = 34.7, so the obesity row applies
Adjusted = 73.18 + 0.4 × (110 − 73.18) = 87.9 kg (headline)
Actual 110.0 kg; ideal 73.2 kg; lean 71.9 kg — all printed beside it
The highest candidate is about 1.5 times the lowest, and a Cockcroft-Gault clearance scales with it
Which weight, by drug, and who says so
| Use | Not obese | Obese | Source |
|---|---|---|---|
| Aminoglycoside, extended interval | Actual | IBW + 0.4 × excess, above 120% of IBW | Nicolau 1995 (Hartford); Bauer 1983 |
| Vancomycin loading dose | Actual | Actual, 20–25 mg/kg, maximum 3,000 mg | Rybak 2020 consensus |
| Cockcroft-Gault | Actual | Disputed: actual, ideal, adjusted or lean | Cockcroft 1976; Winter 2012; Demirovic 2009; institutional protocols |
| Unfractionated heparin | Actual | Disputed: actual (validated), adjusted (suggested), or capped | Raschke 1993; Barletta 2008; Fan 2016 |
Cockcroft-Gault in morbid obesity: how far each weight was off (Demirovic 2009)
| Weight used | Error against measured clearance |
|---|---|
| Actual | Overestimated about two-fold |
| Ideal | Underestimated about 22% |
| Adjusted (0.4) | Overestimated about 30% |
| Lean (Janmahasatian) | Most accurate |
The weight is part of the dose
Every mg/kg dose and every Cockcroft-Gault estimate contains a weight, and in a patient of average build it hardly matters which one. In obesity it matters a great deal: for a man of 178 cm and 110 kg, actual, ideal, adjusted and lean weights run from about 72 to 110 kg, and a clearance calculated on the top one is half as large again as one on the bottom. The choice is not a detail. It is part of the dose.
For two of the four uses here the published guidance is clear. The Hartford once-daily aminoglycoside program dosed on actual weight unless the patient was more than 20% over ideal, and on ideal plus 40% of the excess above that. The 2020 vancomycin consensus bases loading doses on actual weight, obesity included, with a 3,000 mg ceiling. For the other two it is not. For Cockcroft-Gault in obesity, one study found lean weight most accurate and adjusted weight 30% high, another found the adjusted weight the most accurate of the corrections tried, and protocols variously name actual, ideal or adjusted weight. For unfractionated heparin, the nomogram was validated on actual weight, while later obesity studies suggest adjusted weight and many hospitals simply cap the dose.
Where the sources disagree, the page shows a headline so it has something to calculate with, says in words that it is one convention of several, and prints all four candidate weights underneath. It does not settle the disagreement, because nothing published does. The honest instruction is to use the convention your protocol names, the same way every time, and to record it. Drugs without a sourced row — enoxaparin, the direct oral anticoagulants and others — are left out rather than guessed. The ideal body weight calculator, adjusted body weight calculator and lean body weight calculator explain each weight; the Cockcroft-Gault creatinine clearance calculator, vancomycin AUC24 calculator and weight-based heparin nomogram calculator use them.
Frequently asked questions
Should I use actual, ideal or adjusted body weight?
It depends on the drug. For extended-interval aminoglycosides, the Hartford program used actual weight unless the patient was more than 20% over ideal, then ideal + 0.4 × excess. For a vancomycin loading dose the 2020 consensus uses actual weight, capped at 3,000 mg. For Cockcroft-Gault and unfractionated heparin in obesity the published guidance disagrees, and the right answer is the convention your protocol names.
Which weight should I use in Cockcroft-Gault for an obese patient?
There is no consensus. The equation was derived on actual weight, which overestimates clearance about two-fold in morbid obesity. In one study ideal weight underestimated by about 22%, the 0.4-adjusted weight overestimated by about 30%, and lean body weight was most accurate; another found the adjusted weight the most accurate of the corrections tested. Protocols differ. Use yours consistently and record the weight used.
Is the vancomycin loading dose based on actual or ideal body weight?
Actual body weight. The 2020 ASHP/IDSA/PIDS/SIDP consensus bases loading doses on actual weight; in obese patients it suggests 20–25 mg/kg of actual weight with a maximum of 3,000 mg. Maintenance doses in that guideline are adjusted to an AUC target rather than set from a weight.
What weight do I use for a heparin infusion in obesity?
The Raschke nomogram was validated on actual body weight. Studies in obese patients found that produced supratherapeutic aPTTs, and a 2020 critical care pharmacy review suggests adjusted weight instead; many hospitals cap the bolus and starting rate. The subsequent aPTT-guided adjustments correct for the starting weight either way. Follow the local protocol.
Why doesn’t this page cover enoxaparin or the DOACs?
Because no single sourced row could be written for them. Their dosing weight rules come from each drug’s label or trial and differ between drugs, and some labels specify actual body weight for the creatinine clearance calculation. Where a label states which weight to use, the label outranks this page.
Related calculators
References
- 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.
- 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.
- Rybak MJ, Le J, Lodise TP, et al. Therapeutic monitoring of vancomycin for serious methicillin-resistant Staphylococcus aureus infections: a revised consensus guideline and review by the American Society of Health-System Pharmacists, the Infectious Diseases Society of America, the Pediatric Infectious Diseases Society, and the Society of Infectious Diseases Pharmacists. Am J Health Syst Pharm. 2020;77(11):835–864. doi:10.1093/ajhp/zxaa036. Loading doses on actual body weight; in obesity 20–25 mg/kg of actual body weight, maximum 3,000 mg; maintenance dosing guided by AUC.
- Cockcroft DW, Gault MH. Prediction of creatinine clearance from serum creatinine. Nephron. 1976;16(1):31–41.
- Winter MA, Guhr KN, Berg GM. Impact of various body weights and serum creatinine concentrations on the bias and accuracy of the Cockcroft-Gault equation. Pharmacotherapy. 2012;32(7):604–612. Reported the 0.4-adjusted weight as the most accurate of the corrections tested in obese patients.
- 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.
- Raschke RA, Reilly BM, Guidry JR, Fontana JR, Srinivas S. The weight-based heparin dosing nomogram compared with a "standard care" nomogram: a randomized controlled trial. Ann Intern Med. 1993;119(9):874–881. Dosed on actual body weight.
- Fan J, John B, Tesdal E. Evaluation of heparin dosing based on adjusted body weight in obese patients. Am J Health Syst Pharm. 2016;73(19):1512–1522; and Barletta JF, DeYoung JL, McAllen K, et al. Limitations of a standardized weight-based nomogram for heparin dosing in patients with morbid obesity. Surg Obes Relat Dis. 2008;4(6):748–753.
- 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.
- Janmahasatian S, Duffull SB, Ash S, Ward LC, Byrne NM, Green B. Quantification of lean bodyweight. Clin Pharmacokinet. 2005;44(10):1051–1065. Men: LBW = 9270 × weight ÷ (6680 + 216 × BMI); women: LBW = 9270 × weight ÷ (8780 + 244 × BMI).
- 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.
- Stanford Health Care. Aminoglycoside dosing guideline (Antimicrobial Stewardship Program). An institutional protocol: Devine for heights over 60 inches with 45 kg for women; total body weight unless obese, then ideal + 0.4 × excess; Cockcroft-Gault on ideal or adjusted weight; extended-interval dosing avoided in pregnancy, gram-positive synergy, ascites, burns over 20% and creatinine clearance below 30 mL/min or rapidly declining renal function.
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.