How this instrument works
This calculator estimates predicted body weight from height alone, using the Devine formula, then multiplies it by a target of milliliters per kilogram to size a protective tidal volume. Men and women get slightly different constants, but both formulas depend only on height converted to inches past a 60-inch baseline — actual body weight never enters the calculation at all, which is the entire point of using a predicted figure instead of a measured one.
The approach comes out of the landmark ARDSnet trial, published in 2000, which randomized patients with acute lung injury to either a traditional tidal volume of around 12 mL/kg or a lower-volume strategy of 6 mL/kg of predicted body weight, and stopped early because the lower-volume group had a clear mortality advantage. Since then, roughly 4 to 8 mL/kg of predicted body weight has become the commonly used range in lung-protective ventilation, with 6 mL/kg as the figure most directly tested in the original trial.
Sizing to predicted rather than actual weight matters most for patients heavier than their height would suggest: an obese patient's lungs are roughly the same size as a lean person of the same height, so a tidal volume dosed to actual weight would over-stretch alveoli that never grew to match the extra tissue. The Devine formula was chosen for its simplicity and long history in dosing calculations, not because it captures every detail of an individual's actual lung capacity.
- Set Sex — it selects which Devine-formula constant applies to the predicted-weight calculation.
- Enter Height, in any of the offered units; internally the formula converts to inches.
- Set Target volume in mL per kg of predicted body weight, within the commonly used 4-to-8 range.
- Read Predicted body weight (kg) and Tidal volume (mL), both recalculated as any field changes.
Worked example — 1.75 m man at 6 mL/kg
A man measuring 1.75 m (about 68.9 inches) has a predicted body weight of 50 + 2.3 × (68.9 − 60) ≈ 50 + 20.46 = 70.46 kg — notice his actual weight never enters this at all. At the ARDSnet-tested target of 6 mL/kg, tidal volume works out to 6 × 70.46 ≈ 422.8 mL.
A 1.60 m woman uses the same arithmetic with the female constant: 45.5 + 2.3 × (63.0 − 60) ≈ 52.38 kg predicted body weight, giving 6 × 52.38 ≈ 314.3 mL at the same 6 mL/kg target. A taller, 1.90 m man at the upper end of the protective range, 8 mL/kg, comes out to 84.05 kg predicted weight and 8 × 84.05 ≈ 672.4 mL — nearly 60% more breath than the shortest patient here, entirely from height and target, never from body fat.
Questions
Why does this calculator ignore my actual body weight?
Because lung size tracks height, not body weight. A taller person has bigger lungs regardless of how much they weigh, and a heavier person's lungs haven't grown to match extra body fat. Dosing tidal volume to actual weight in a larger patient would over-inflate lungs sized for a much smaller frame, which is exactly what the ARDSnet trial's protocol was designed to avoid.
What tidal volume range is considered lung-protective?
Roughly 4 to 8 mL per kilogram of predicted body weight, with 6 mL/kg being the specific figure tested in the original 2000 ARDSnet trial that first showed a mortality benefit from lower volumes. Higher figures within that range are sometimes used depending on a patient's underlying lung mechanics and clinical judgment.
Why does the formula use 60 inches as a starting point?
It's simply the reference height the Devine formula was built around — every inch of height above five feet adds 2.3 kg to the predicted weight, and every inch below it subtracts the same amount. It's an empirical fit from decades of clinical dosing use, not a physiological law.
Is predicted body weight the same as ideal body weight?
They're closely related concepts calculated the same way, but 'predicted body weight' is the specific term used in ventilator-management literature, chosen because tidal volume dosing isn't about an 'ideal' weight for health so much as a reasonable estimate of lung size from height.
Does a bigger tidal volume always mean better ventilation?
No — the entire finding behind lung-protective ventilation is the opposite. Larger breaths can over-stretch already-injured lung tissue and worsen outcomes, which is why the ARDSnet trial's lower-volume arm, dosed to predicted rather than actual weight, showed a survival advantage over the older, larger-volume approach.
References
- ARDSnet (Brower et al.) 2000, NEJM — lower tidal volumes trial (PubMed)
- Predicted body weight formulas for protective ventilation (PMC)
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.