Which weight to use for dosing

Determine whether a given drug should be dosed on total, ideal, or adjusted body weight for a specific patient, and return the resulting dosing weight.

High-consequence calculationSelecting the wrong dosing weight is a leading cause of aminoglycoside and other narrow-therapeutic-index dosing errors. Always confirm against the product labelling and your institutional protocol.

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What is this for?

Most drugs are dosed on total body weight, including in obesity. A short list of exceptions distribute poorly into adipose tissue and are dosed on ideal body weight, while aminoglycosides in obesity use adjusted body weight. This tool applies the rules to a specific patient and returns the number you would actually use.

How to use it

Enter the patient's sex, height, and actual weight.
Select the drug. If the agent is not one of the listed exceptions, leave the selection on "Any other drug".
The result gives the weight category and the number to use. Cross-check the reference table below the result.

Worked example

A 175 cm, 130 kg male (BMI 42.4) is starting gentamicin. Which weight should the dose be based on?

Answer: IBW = 50 + 2.3 × 8.9 = 70.5 kg. BMI 42.4 means obese, so aminoglycosides use AdjBW = 70.5 + 0.4 × (130 − 70.5) = 94.3 kg.

Clinical pearls & pitfalls

The default is total body weight. The exceptions are a short list, and treating IBW as the default is a common and consequential error that underdoses patients.
If the patient weighs less than IBW, use total body weight regardless of the drug. IBW is never a floor.
Vancomycin uses total body weight even in obesity — it is frequently and wrongly assumed to follow the aminoglycoside rule.
Hydrophilic drugs tend toward IBW or AdjBW; lipophilic drugs tend toward TBW. That principle helps predict the rule but does not replace checking the specific agent.

Assumptions & limitations

Covers only the widely taught exceptions. Many drugs have agent-specific dosing-weight guidance in their labelling that this list does not reproduce.
The obesity determination uses BMI ≥ 30. Some references instead use a percentage of ideal body weight, typically 120% or 130%, which can classify a borderline patient differently.
Provides a dosing weight, not a dose. The mg/kg figure, frequency, and renal adjustment all still have to be applied.

References

Pai MP, Bearden DT. Antimicrobial dosing considerations in obese adult patients. Pharmacotherapy. 2007;27(8):1081-1091.
Erstad BL. Dosing of medications in morbidly obese patients in the intensive care unit setting. Intensive Care Med. 2004;30(1):18-32.
Rybak MJ, et al. Therapeutic monitoring of vancomycin for serious MRSA infections: a revised consensus guideline. Am J Health Syst Pharm. 2020;77(11):835-864.