Which ideal body weight formula should I use for drug dosing?

The Devine formula (1974) is the most widely used for drug dosing, appearing in most pharmacokinetic studies and drug labelling. It is the default for aminoglycoside dosing, ventilator tidal volume settings, and Cockcroft-Gault calculations. The Robinson formula is considered slightly more accurate statistically but Devine remains the clinical standard.

How is adjusted body weight (ABW) calculated?

Adjusted body weight = IBW + 0.4 × (Actual body weight − IBW). The correction factor of 0.4 accounts for partial drug distribution into adipose tissue. ABW is commonly used for aminoglycoside dosing and Cockcroft-Gault CrCl estimation in obese patients.

Ideal Weight Calculator – Devine, Robinson, Miller, Hamwi, Peterson

Q: What is ideal body weight (IBW)?

Ideal body weight is the estimated optimal body weight for a person's height and sex, independent of actual body composition. It is used primarily as a reference weight for drug dosing in pharmacokinetics, tidal volume calculation in mechanical ventilation, and as a component of adjusted body weight in obese patients.

What Is Ideal Body Weight (IBW)?

Ideal body weight is an estimated weight at which health risk is minimised for a given height and sex. In clinical practice, IBW is not primarily used as a health or aesthetic target — it is a pharmacokinetic reference weight used for drug dosing, tidal volume setting in mechanical ventilation, and as a component of adjusted body weight calculation.

The Five IBW Formulas Compared

Formula	Year	Male Equation	Female Equation
Devine ✦	1974	50 + 2.3 × (H_in − 60)	45.5 + 2.3 × (H_in − 60)
Robinson	1983	52 + 1.9 × (H_in − 60)	49 + 1.7 × (H_in − 60)
Miller	1983	56.2 + 1.41 × (H_in − 60)	53.1 + 1.36 × (H_in − 60)
Hamwi	1964	48 + 2.7 × (H_in − 60)	45.4 + 2.3 × (H_in − 60)
Peterson	2016	2.2 × BMI + 3.5 × BMI × (H_m − 1.5)	Same equation

H_in = height in inches above 5 feet (60 inches); H_m = height in metres. ✦ Recommended for drug dosing.

Adjusted Body Weight (ABW)

For obese patients (actual weight > IBW), many drugs distribute partially into adipose tissue. Adjusted body weight accounts for this:

ABW = IBW + 0.4 × (Actual weight − IBW)
The 0.4 correction factor is a general approximation; some drugs use different factors (e.g. 0.3 for vancomycin in some protocols).
ABW is used for: aminoglycoside dosing, Cockcroft-Gault CrCl in obese patients, nutritional support calculations, and mechanical ventilation tidal volume.

IBW in Mechanical Ventilation

Protective lung ventilation uses IBW (not actual weight) to set tidal volume targets: 6–8 mL/kg IBW. Using actual weight in obese patients would over-ventilate and increase barotrauma risk. The Devine formula is standard for this calculation.

Frequently Asked Questions

Which IBW formula should I use for gentamicin or vancomycin dosing?+

The Devine formula is standard for aminoglycoside (gentamicin, tobramycin) dosing. For obese patients, use adjusted body weight (ABW = IBW + 0.4 × [ABW − IBW]) rather than actual body weight, as these drugs do not distribute significantly into fat tissue. For vancomycin, most protocols use actual body weight for the loading dose; consult your local protocol.

What if my actual weight is below my IBW?+

If actual weight is below the calculated IBW, use actual body weight for dosing rather than IBW. The IBW formulas are designed to provide an upper reference — they are not intended to suggest a minimum acceptable weight. For very underweight patients, drug dosing should be reviewed carefully by a clinical pharmacist as pharmacokinetics may be significantly altered.

Why does the Peterson formula give different results from the others?+

The Peterson formula (2016) uses a different approach — it derives IBW from a target BMI range rather than a fixed height-based equation. This makes it sex-neutral (the same equation applies to both males and females using actual BMI) and theoretically more physiologically grounded. However, it is the least validated in drug dosing studies compared to Devine, so clinical uptake has been limited.

Are IBW formulas accurate for very tall or very short people?+

The traditional IBW formulas (Devine, Robinson, Miller, Hamwi) were derived from height ranges of approximately 152–193 cm (5'0"–6'4"). They become progressively less reliable outside this range. For very tall individuals, IBW may be underestimated; for very short adults or children, the formulas may give physiologically implausible results. The Peterson formula performs somewhat better at extremes of height.

Related Tools

For BMI-based weight assessment see BMI Calculator. For BSA-based drug dosing see BSA Calculator. For weight-based and BSA drug dose calculation see Weight-Based Dose Calculator. For CrCl estimation using IBW in obese patients see Creatinine Clearance.

References

Devine BJ. "Gentamicin therapy." Drug Intelligence and Clinical Pharmacy. 1974;8:650-655.
Robinson JD, et al. "Determination of ideal body weight for drug dosage calculations." Am J Hosp Pharm. 1983;40(6):1016-1019.
Miller DR, et al. "Determination of ideal body weight for drug dosage." Am J Hosp Pharm. 1983;40(3):468.
Hamwi GJ. Therapy: Changing Dietary Concepts. American Diabetes Association, 1964.
Peterson CM, et al. "Universal equation for estimating ideal body weight." Am J Clin Nutr. 2016;103(5):1197-1203.

⚠️ Medical Disclaimer: Ideal weight calculations are reference tools for clinical pharmacokinetics and are not health targets or aesthetic standards. All drug dosing decisions must be verified by a qualified pharmacist or prescriber.