What is ideal body weight used for in drug dosing?

IBW is used for drug dosing when the drug does not distribute significantly into adipose (fat) tissue. Using actual body weight in obese patients for such drugs would lead to overdose. Common drugs dosed on IBW include aminoglycosides (gentamicin, tobramycin) and drugs where volume of distribution is not affected by fat mass.

What is adjusted body weight and when is it used?

Adjusted body weight (ABW) = IBW + 0.4 x (actual weight - IBW). It is used for drug dosing in obese patients when the drug partially distributes into fat tissue. ABW is a compromise between actual and ideal body weight. It is used for creatinine clearance calculation in obese patients and for some drug dosing calculations.

What is the Devine formula for ideal body weight?

The Devine formula calculates IBW as: Male IBW = 50 kg + 2.3 kg per inch over 5 feet. Female IBW = 45.5 kg + 2.3 kg per inch over 5 feet. It is the most widely used IBW formula in clinical practice, particularly for drug dosing.

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Ideal body weight calculator

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Calculate ideal body weight (IBW) and adjusted body weight (ABW) using 4 validated formulas: Devine, Hamwi, Robinson and Miller. Used for drug dosing weight selection in obese patients and Cockcroft-Gault CrCl calculation.

Sex

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Height

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Actual body weight (kg, optional for ABW)

Required to calculate adjusted body weight (ABW).

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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.

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.

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. 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.

Frequently asked questions

What is ideal body weight used for?

IBW is the dosing weight for drugs that don't distribute into adipose tissue in obese patients. Most commonly used for aminoglycoside antibiotics (gentamicin, tobramycin, amikacin) and in Cockcroft-Gault CrCl calculation when actual weight exceeds IBW. If actual weight is below IBW, use actual weight.

What is adjusted body weight?

ABW = IBW + 0.4 × (actual weight - IBW). Used for drugs that partially distribute into adipose tissue in patients where actual weight exceeds IBW by more than 30%. The 0.4 factor represents approximately 40% distribution into fat tissue. Confirm the appropriate weight type with a pharmacist for your specific drug.

Which IBW formula should I use?

Devine is the default for clinical drug dosing because most drug dosing guidelines that specify IBW were developed using the Devine formula. Hamwi is widely used in nutrition practice. Robinson and Miller are alternatives with slightly different base weights. When a protocol specifies IBW without naming a formula, Devine is assumed.