In an obese patient, choosing the right weight to dose by is often more important than the dose equation itself. A drug that stays in lean tissue must not be dosed on total body weight, and a drug that spreads into fat must not be dosed on ideal weight. This selector computes every relevant weight and recommends the one your drug class needs.
How it works
The tool first finds BMI and the four body weights. Ideal body weight comes from the Devine formula:
IBW (male) = 50.0 + 2.3 x (height_in - 60)
IBW (female) = 45.5 + 2.3 x (height_in - 60)Adjusted body weight corrects ideal weight upward by a fraction of the excess, using the standard correction factor of 0.4:
AdjBW = IBW + 0.4 x (ABW - IBW)Lean body weight uses the Janmahasatian equation, which folds in BMI so it behaves sensibly at the extremes:
LBW (male) = (9270 x ABW) / (6680 + 216 x BMI)
LBW (female) = (9270 x ABW) / (8780 + 244 x BMI)Each drug class is mapped to one of these weights based on how the drug distributes and clears.
Worked example and notes
A 110 kg, 175 cm male has a BMI of about 35.9, placing him in obesity class II. His ideal weight is roughly 70.6 kg and his adjusted weight roughly 86.4 kg. For an aminoglycoside the tool recommends adjusted body weight, so the dose is calculated on about 86 kg rather than the full 110 kg, avoiding a large overdose of a drug confined to lean tissue.
These mappings are general guidance defaults. The correct dosing weight can change with renal function, the specific indication, and therapeutic drug monitoring, and local protocols sometimes differ. Always confirm against the product information and a clinical pharmacist before prescribing.
Drug-by-drug dosing weight summary
The following table summarises the commonly accepted dosing weight for several frequently encountered drug classes in obese patients. This is general guidance; always verify against product information and local protocol.
| Drug / class | Recommended dosing weight | Rationale |
|---|---|---|
| Aminoglycosides (gentamicin, tobramycin) | Adjusted body weight (AdjBW) | Water-soluble; distributes primarily in lean tissue, but some distribution into adipose |
| Vancomycin (AUC-guided) | Actual body weight (ABW) | Distributes proportionally with total body weight; dose on ABW then target AUC 400–600 |
| Low-molecular-weight heparins (enoxaparin, prophylaxis) | ABW up to a cap (often 100–150 kg depending on agent) | Direct relationship with ABW, but very high doses unvalidated |
| Low-molecular-weight heparins (treatment dose) | ABW (with anti-Xa monitoring) | Anti-Xa levels guide adjustment |
| Unfractionated heparin | ABW | Weight-based protocol with aPTT monitoring |
| Neuromuscular blockers (rocuronium, vecuronium) | Ideal body weight (IBW) | Effect site is lean muscle; ABW dosing prolongs neuromuscular block |
| Succinylcholine | Actual body weight | Pseudocholinesterase activity increases with body mass |
| Propofol (induction) | Lean body weight (LBW) | Distribution into lean tissue; ABW overdoses |
| Propofol (maintenance infusion) | ABW | Propofol redistributes into adipose during prolonged infusion |
| Midazolam (acute sedation) | IBW for loading dose | Lipophilic but IBW for initial dose reduces over-sedation; ABW for prolonged infusions |
| Phenytoin | IBW + 1.33 × (ABW − IBW) | Distributes into lean tissue with a partial adipose correction |
Why the Janmahasatian lean body weight equation is preferred
An older LBW equation (James formula) has a known flaw: it produces illogical results at high BMI values, eventually turning negative as BMI increases beyond roughly 37 (men) or 32 (women). The Janmahasatian formula used here (LBW = (9270 × ABW) / (6680 + 216 × BMI) for males) behaves correctly at any BMI, which is precisely when an accurate LBW matters most. If your institution’s pharmacy system uses the James formula, be aware of this limitation when applying LBW-based doses to class II and class III obesity patients.