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Calculate ideal body weight (IBW) with the Devine formula, check the 120% IBW threshold, and get adjusted body weight (ABW) in kg and lb: ABW = IBW + 0.4 × (actual − IBW). Built for pharmacy, nursing, and medical students learning obesity dosing — always verify the specific drug monograph.
Last updated: June 4, 2026
IBW
73.0 kg
Actual
99.8 kg
% of IBW
137%
ABW
83.7 kg
Above 120% IBW → ABW = 73.0 + 0.4 × (99.8 − 73.0) ≈ 83.7 kg
| Clinical situation | Weight for dosing | ABW role |
|---|---|---|
| Actual weight < 120% of IBW | Usually actual body weight or IBW per drug monograph | ABW not routinely indicated |
| Actual weight ≥ 120% of IBW (obese by IBW ratio) | Often adjusted body weight (if drug specifies) | ABW = IBW + 0.4 × (actual − IBW) |
| Actual weight < IBW (underweight) | Typically actual body weight | Do not use ABW; IBW overestimates needs |
| Lipophilic drugs (many sedatives, some chemo) | May use actual TBW or other rules — not ABW | Check monograph; ABW is for hydrophilic agents |
If actual weight ≥ 1.2 × IBW, proceed with ABW for drugs that specify it. If below, use actual or IBW per monograph.
| Factor / weight | Typical use |
|---|---|
| 0.3 | Some institutional protocols / specific drugs |
| 0.4 | Most common default (aminoglycosides, many teaching examples) |
| 0.5 | Occasionally cited; verify before use |
| 1.0 (actual) | Hydrophilic drugs when not obese by IBW ratio |
| IBW only | Some weight-based drugs in non-obese patients |
| Drug / setting | Weight basis | Notes |
|---|---|---|
| Aminoglycosides (e.g., gentamicin, tobramycin) | Often ABW when actual > 120% IBW | Hydrophilic; Vd affected by excess adipose — classic ABW teaching case |
| Some IV antimicrobials / high Vd agents | ABW or IBW per protocol | Hospital antibiogram and renal function (CrCl) still required |
| Chemotherapy (BSA-based) | Body surface area, not ABW | Use our BSA calculator — different pathway |
| Heparin / many anticoagulants | Actual weight common | ABW usually not applied |
IBW = 50 + 23 = 73 kg
136% of IBW → use ABW
ABW ≈ 87.8 kg
IBW = 45.5 + 9.2 = 54.7 kg
168% of IBW
ABW ≈ 69.6 kg
IBW ≈ 68.4 kg; 120% IBW = 82.1 kg
Actual below threshold
ABW not routinely used
Not prescribing advice. This calculator supports learning and double-checks. Licensed clinicians and pharmacists must confirm drug-specific weight rules, renal/hepatic function, and therapeutic drug monitoring before administration.
For pharmacy students, nurses, and clinicians reviewing ABW and Devine IBW
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