Ideal body weight (IBW) is a height-based weight estimate that clinicians use when total body weight would give the wrong answer. Many drugs distribute mainly into lean tissue rather than fat, and modern ventilation targets tidal volumes scaled to a patient’s predicted lean size, so a reliable IBW figure underpins safe dosing and ventilator management.
How it works
Both formulas take height in inches and build up from a sex-specific base weight at 60 inches (152.4 cm):
Devine male: 50.0 + 2.3 x (height_in - 60)
Devine female: 45.5 + 2.3 x (height_in - 60)
Robinson male: 52.0 + 1.9 x (height_in - 60)
Robinson female: 49.0 + 1.7 x (height_in - 60)The tool accepts height in centimetres and converts to inches before applying the equations. Below 60 inches the estimate is floored to the base weight.
Worked examples
Example 1 — Male, 175 cm: 175 cm converts to about 68.9 inches.
- Devine IBW:
50 + 2.3 × (68.9 − 60) = 50 + 20.5 = 70.5 kg - Robinson IBW:
52 + 1.9 × (68.9 − 60) = 52 + 16.9 = 68.9 kg
Example 2 — Female, 162 cm: 162 cm converts to about 63.8 inches.
- Devine IBW:
45.5 + 2.3 × (63.8 − 60) = 45.5 + 8.7 = 54.2 kg - Robinson IBW:
49 + 1.7 × (63.8 − 60) = 49 + 6.5 = 55.5 kg
Use IBW for lung-protective tidal volume targets and for hydrophilic drug dosing, and switch to an adjusted body weight in markedly obese patients where ideal weight would under-dose.
Clinical applications and limitations
Mechanical ventilation
In acute respiratory distress syndrome (ARDS) management, lung-protective ventilation targets a tidal volume of 6 mL/kg of IBW — sometimes 4–8 mL/kg depending on lung compliance. Basing tidal volume on actual body weight in an obese patient would over-distend already-injured lungs, since the lung size tracks height, not body mass. IBW provides the height-scaled reference the lungs actually require.
Drug dosing
Hydrophilic drugs — aminoglycosides (gentamicin, tobramycin), vancomycin — distribute primarily into lean tissue and plasma water rather than adipose tissue. For these agents, using actual body weight in an obese patient results in an overdose because the drug does not distribute into the extra fat mass. IBW gives a better approximation of the volume of distribution for these medications.
However, IBW is not universally superior. Some drugs use actual body weight, lean body weight (a different estimate), or an adjusted body weight that interpolates between IBW and actual body weight for obese patients. The Devine IBW is the default for many drug-dosing references, so checking the specific dosing guidance is essential rather than assuming IBW applies to every medication.
Obesity consideration
When actual body weight significantly exceeds IBW — typically defined as more than 20–30% above it — an adjusted body weight is frequently used instead:
adjusted body weight = IBW + 0.4 × (actual weight − IBW)This interpolation reflects partial drug distribution into fat tissue for agents with intermediate volume-of-distribution characteristics. The adjusted body weight calculator (linked in related tools) handles this calculation.
Patients below 60 inches
The Devine and Robinson formulas were validated primarily for adults of at least 60 inches (152.4 cm). For shorter patients the linear formula produces IBWs at or below the base weight, which this tool floors at the base to avoid biologically unreasonable values. For very short patients, clinical judgment and alternative references should supplement this estimate.