Three weights for dosing
Doses of many drugs and nutrition plans are based on body weight, but in a person with obesity, extra fat tissue does not take up most drugs the way muscle and organs do. Clinicians therefore use three weights:
- Ideal body weight (IBW): an estimate based on height and sex only.
- Adjusted body weight (ABW): ideal weight plus 40% of the excess weight.
- Lean body weight (LBW): weight minus fat mass.
The formulas
- IBW (Devine): men 50 kg + 2.3 kg per inch over 60 in; women 45.5 kg + 2.3 kg per inch over 60 in
- ABW = IBW + 0.4 × (actual weight − IBW)
- LBW (Janmahasatian): men 9,270 × weight ÷ (6,680 + 216 × BMI); women 9,270 × weight ÷ (8,780 + 244 × BMI)
If actual weight is below ideal weight, the calculator shows the actual weight as the adjusted weight, since there is no excess to adjust.
Worked example
A man is 5 ft 10 in (70 in) tall and weighs 250 lb (113.4 kg).
- IBW = 50 + 2.3 × 10 = 73 kg (161 lb).
- Excess = 113.4 − 73 = 40.4 kg.
- ABW = 73 + 0.4 × 40.4 = 89.2 kg (196.6 lb).
- Actual weight is 155% of ideal weight.
- BMI = 113.4 ÷ 1.778² = 35.9, and LBW = 9,270 × 113.4 ÷ (6,680 + 216 × 35.9) = 72.9 kg.
Which weight for which use
| Use | Weight usually chosen |
|---|---|
| Aminoglycosides (gentamicin, tobramycin) in obesity | Adjusted weight |
| Creatinine clearance (Cockcroft-Gault) when weight is above 120% of IBW | Adjusted weight, sometimes ideal weight |
| Tidal volume on a ventilator | Ideal (predicted) weight |
| Propofol induction, some opioids | Lean body weight |
| Low molecular weight heparin, many antibiotics | Actual weight, often with a cap |
These are common patterns, not rules: always follow the drug label, the hospital protocol or a clinical pharmacist.
Limits
The Devine formula was published in 1974 to dose one drug and was never validated as a measure of healthy weight. It works poorly for people shorter than 5 ft, for whom it can give very low or even negative values. The 0.4 factor in the adjusted weight comes from studies of aminoglycosides, and other drugs may need a different fraction.
This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.