Educational tool — not a treatment guide. Ventilator settings are prescribed and adjusted by trained clinicians using the whole clinical picture: plateau and driving pressures, PEEP, gas exchange, haemodynamics and patient synchrony.
This page implements a published formula for teaching and reference. It must not be used to set or change ventilation for any patient.
Quick answer: tidal volume is set from predicted body weight, calculated from height and sex, never from actual weight. Male PBW = 50 + 2.3 × (height in inches − 60); female uses 45.5. Multiply PBW by the target — the ARDSNet protocol uses 6 mL/kg, adjustable between 4 and 8.
Why predicted, not actual, body weight
This is the single most important idea on the page. Lungs do not grow when a person gains weight. Lung size is determined largely by height and sex, both fixed in adulthood — so a 170 cm patient has roughly the same lung capacity at 60 kg as at 120 kg.
Setting tidal volume from actual weight in an obese patient would therefore deliver a breath far too large for their lungs. Overdistension of that kind is precisely the mechanism — ventilator-induced lung injury — that lung-protective ventilation was developed to avoid. Predicted body weight is the standard denominator for exactly this reason.
| Height | Male PBW | Female PBW | Male Vt at 6 mL/kg | Female Vt at 6 mL/kg |
|---|---|---|---|---|
| 150 cm (4'11") | 47.8 kg | 43.3 kg | 287 mL | 260 mL |
| 155 cm (5'1") | 52.4 kg | 47.9 kg | 314 mL | 287 mL |
| 160 cm (5'3") | 56.9 kg | 52.4 kg | 341 mL | 314 mL |
| 165 cm (5'5") | 61.4 kg | 56.9 kg | 368 mL | 341 mL |
| 170 cm (5'7") | 65.9 kg | 61.4 kg | 396 mL | 369 mL |
| 175 cm (5'9") | 70.5 kg | 66.0 kg | 423 mL | 396 mL |
| 180 cm (5'11") | 75.0 kg | 70.5 kg | 450 mL | 423 mL |
| 185 cm (6'1") | 79.5 kg | 75.0 kg | 477 mL | 450 mL |
| 190 cm (6'3") | 84.0 kg | 79.5 kg | 504 mL | 477 mL |
The formulas
Female PBW (kg) = 45.5 + 2.3 × (height in inches − 60)
Tidal volume = PBW × target (mL/kg)
Worked example — a male patient 175 cm tall:
- Convert height: 175 cm ÷ 2.54 = 68.9 inches
- PBW: 50 + 2.3 × (68.9 − 60) = 50 + 20.5 = 70.5 kg
- At 6 mL/kg: 70.5 × 6 = 423 mL
- Range at 4–8 mL/kg: 282 mL to 564 mL
The predicted body weight equations are those of Devine (1974), adopted by the ARDS Network. The 6 mL/kg target and 4–8 mL/kg adjustment range come from the ARDSNet lung-protective ventilation protocol, established by the ARMA trial published in the New England Journal of Medicine in 2000.
Targets across the range
| Target | Typical context |
|---|---|
| 4 mL/kg PBW | Lower limit of the ARDSNet range, used when plateau pressure remains high |
| 6 mL/kg PBW | The ARDSNet default target for ARDS |
| 8 mL/kg PBW | Upper limit of the range; also common for lungs without ARDS |
| Above 8 mL/kg | Historically routine, now generally avoided as injurious |
Tidal volume is never adjusted in isolation. The ARDSNet protocol pairs its 6 mL/kg target with a plateau pressure limit, reducing tidal volume further if that limit is exceeded, and raising respiratory rate to compensate for the reduced minute ventilation. Driving pressure and PEEP form part of the same strategy. The number this calculator produces is a starting point within a protocol, not a setting on its own.
Frequently asked questions
How is tidal volume calculated?
From predicted body weight, not actual weight. First calculate PBW from height and sex using the Devine formula, then multiply by the target in millilitres per kilogram. A 175 cm male has a PBW of about 70.5 kg, so a 6 mL/kg target gives roughly 423 mL.
Why use predicted body weight instead of actual weight?
Because lung size correlates with height and sex, not with body fat. A patient who gains 30 kg does not gain lung capacity. Setting tidal volume from actual weight in an obese patient would deliver a dangerously large breath relative to their lungs, which is exactly the injury lung-protective ventilation exists to prevent.
What is the ARDSNet tidal volume target?
The ARDSNet protocol targets 6 mL per kilogram of predicted body weight, adjustable within a 4 to 8 mL/kg range according to plateau pressure and patient synchrony. The landmark ARMA trial published in 2000 found lower mortality with 6 mL/kg compared with the 12 mL/kg then in common use.
What is the Devine formula for predicted body weight?
For males, PBW in kilograms = 50 + 2.3 × (height in inches − 60). For females, PBW = 45.5 + 2.3 × (height in inches − 60). In metric terms that becomes 50 or 45.5 plus 0.91 kg for every centimetre above 152.4 cm.
Does tidal volume alone determine lung protection?
No. Plateau pressure, driving pressure, PEEP and respiratory rate all matter alongside it. The ARDSNet protocol pairs the 6 mL/kg target with a plateau pressure limit, and tidal volume is reduced further if that limit is exceeded. A number from a calculator is a starting point for a ventilator strategy, never the whole of it.
References
- Devine BJ. Gentamicin therapy. Drug Intelligence and Clinical Pharmacy, 1974 — origin of the predicted body weight equations.
- The Acute Respiratory Distress Syndrome Network. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. New England Journal of Medicine, 2000;342:1301–1308 (the ARMA trial).
- ARDSNet mechanical ventilation protocol — 6 mL/kg PBW target with 4–8 mL/kg adjustment range and plateau pressure limit.