What is ideal body weight?
Ideal body weight (IBW) is a height-based estimate of a person’s expected lean weight — what they would weigh at a healthy body composition. It deliberately leaves out excess fat so weight-based calculations track lean tissue and lung size rather than adipose mass.
Many bedside numbers scale to lean tissue or lung volume, both of which track height, not fat. In an overweight patient, using total body weight overshoots — too large a tidal volume, too big a drug dose. Ideal body weight keeps these calculations matched to the physiology they target.
Calculating ideal body weight in adults: the Devine formula
For adults, ideal body weight is estimated from height with the Devine formula: men = 50 kg + ~2.3 kg per inch over 5 ft; women = 45.5 kg + ~2.3 kg per inch over 5 ft. Devine first created it for gentamicin dosing, and it has since become the de facto standard for height-based IBW.
Tidal volume: 6–8 mL/kg of ideal body weight
Lung size is set by height, not body fat, so tidal volume is dosed on ideal body weight (predicted body weight), never actual weight. The ARDSNet trial showed 6 mL/kg of predicted weight cut mortality versus 12 mL/kg. Routine intraoperative lung-protective targets are 6–8 mL/kg IBW.
Dosing drugs by ideal, adjusted, or total body weight
The right weight scalar depends on a drug’s fat solubility and volume of distribution. Ideal or lean weight suits non-depolarizing neuromuscular blockers; adjusted body weight (IBW + 0.4 × [actual − IBW]) suits many maintenance infusions and hydrophilic drugs in obesity; total or lean body weight suits succinylcholine and propofol induction.
Ideal body weight dosing is not automatically “ideal” — dosed on IBW alone, some agents are underdosed — so the safest approach weighs ideal, adjusted, and lean body weight together and chooses per drug.
Ideal body weight in children: median weight for age
Children have no height-based Devine equation. Their ideal weight is the sex-specific 50th-percentile (median) weight for age, taken from WHO Child Growth Standards (birth–24 months) and CDC 2000 growth charts (2–17 years).
In non-obese children, actual weight tracks the median, so most doses are per actual kilogram; tidal volume still follows 6–8 mL/kg of ideal weight, with actual weight used under 2 years. In obese children the two diverge, and ideal body weight prevents the overdose that total weight would produce.
Why getting the weight right matters
A wrong weight propagates. An oversized tidal volume risks ventilator-induced lung injury; an overscaled drug dose risks toxicity. Anchoring ventilation and dosing to a single, referenced ideal body weight is one of the simplest ways to make an anesthetic safer — but deriving IBW by hand, then re-deriving every number that depends on it, is exactly the kind of step that gets skipped or fumbled under time pressure.
Calculate ideal body weight instantly with Master Anesthesia
The Master Anesthesia app turns all of the above into a one-step lookup. Enter the patient once — height and sex for an adult, age and sex for a child — and it returns ideal body weight and carries that number straight into the calculations that depend on it: tidal volume at 6–8 mL/kg of IBW, adjusted body weight for the drugs that need it in obesity, and weight-based drug doses with the right scalar flagged per drug, alongside lean body weight, BSA, and BMI on the same screen.
For an obese patient this is where it earns its keep: total body weight would overshoot both the ventilator and several drug doses, and the app keeps every calculation matched to the physiology it targets — instantly, and consistently referenced to the same defensible ideal body weight.