Ideal and adjusted body weight: when and why they matter for drug dosing
Most drugs use actual body weight for dosing. But for some drugs, using actual weight in obese patients leads to overdose because the drug doesn't distribute into adipose tissue. This is where IBW and ABW become clinically essential. A retrospective study of aminoglycoside dosing in morbidly obese patients found the commonly used 40% adjustment factor produced an appropriate drug level in most cases, lending some real-world weight to the ABW formula below.
The Devine formula (default)
Male: IBW = 50 + 2.3 × (height in inches - 60). Female: IBW = 45.5 + 2.3 × (height in inches - 60). Developed in 1974, it's the most widely used formula in clinical practice and the one most drug dosing guidelines reference when they specify IBW, a clinical history documented in Pai and Paloucek's review of the origin of IBW equations.
When to use IBW
Use IBW as the dosing weight for drugs that distribute primarily in lean tissue: aminoglycoside antibiotics (gentamicin, tobramycin, amikacin) in obese patients, some chemotherapy agents and Cockcroft-Gault CrCl calculation in obese patients. Not sure which class a drug you're checking belongs to, or whether it's fat-soluble? Our Drug Class Lookup is a quicker starting point than guessing. If actual weight is less than IBW, always use actual weight.
Adjusted body weight
ABW = IBW + 0.4 × (actual weight - IBW). Used when actual weight exceeds IBW by more than 30%. Some drugs partially distribute into adipose tissue, ABW accounts for this partial distribution. Aminoglycosides in morbidly obese patients are the classic example. Always confirm the appropriate dosing weight with a pharmacist for your specific drug. Then calculate the actual dose using our Weight-Based Dosage Calculator. For renal dosing, pair IBW with the Creatinine Clearance Calculator which uses IBW in obese patients.