π¬οΈ Tidal Volume Calculator
Lung-protective volumes from predicted body weight.
Target tidal volume (6 mL/kg PBW)
For informational purposes only, not medical advice.
Lung-protective tidal volume is 6 to 8 mL per kilogram of predicted body weight, never actual weight. A 178 cm man has a predicted weight of 73.3 kg, so 6 mL/kg gives a target of 440 mL per breath. The ARDSNet trial showed 6 mL/kg beats 12 mL/kg on survival, which made predicted-weight dosing one of the most consequential formulas in critical care.
Lungs come in the size of the skeleton, not the size of the belly. Ventilating an obese patient by actual weight delivers volumes their lungs were never built for, which is exactly the injury the formula prevents.
How to Use This Calculator
Enter height in centimeters and sex. The tool computes predicted body weight with the ARDSNet equations and returns the 6, 7, and 8 mL/kg volumes.
Enter the patient's height in centimeters. Divide inches by 0.3937 if needed.
Select sex, which sets the base constant: 50 kg for men, 45.5 kg for women at 152.4 cm.
Read the predicted weight and the three volumes. Start at 6 mL/kg for ARDS; 8 is the usual upper boundary.
The ARDSNet Formulas
Predicted body weight estimates lean lung-sized mass from height alone. The 0.91 slope adds just under a kilo per centimeter above five feet. It is not the patient's actual weight, ideal weight by insurance tables, or adjusted weight; it is its own purpose-built number, and substituting any other weight is the classic error.
The 6 mL/kg target comes from the landmark 2000 ARDSNet trial: mortality 31 percent versus 39.8 percent against 12 mL/kg. Few ventilator settings have that kind of evidence, which is why lung-protective volume is now the default, not the exception.
Worked Example
A 178 cm man: predicted weight is 50 plus 0.91 times (178 minus 152.4), which is 50 plus 23.3, or 73.3 kg. At 6 mL/kg the tidal volume is 73.3 times 6, about 440 mL. At 8 mL/kg it would be 586 mL.
If this patient actually weighs 120 kg, the contrast is stark: 6 mL/kg of actual weight would be 720 mL, nearly double the lung-protective target. That 720 mL breath is what the predicted-weight formula exists to forbid.
Reading Your Result
| Setting | When |
|---|---|
| 6 mL/kg PBW | ARDS and the standard lung-protective target. |
| 6 to 8 mL/kg | Acceptable range; many clinicians start at 6 and adjust to comfort and plateau pressure. |
| Above 8 mL/kg | Generally avoided; raises volutrauma risk without proven benefit. |
Volume is half the lung-protective strategy; the other half is pressure, keeping plateau pressure under 30 cm H2O. My take: set the volume by this calculator, then let the plateau pressure have the final vote.
Limits
Predicted weight equations were validated in adults and fit poorly at extremes of height. Severe obstructive disease, pregnancy, and raised intracranial pressure all modify targets beyond simple volume math. Ventilator management is ICU specialist work; this page teaches the arithmetic behind one setting.
Related Tools
Lung mechanics pair with oxygenation: the VO2 max calculator covers exercise capacity, the BMI calculator shows why actual weight misleads, and the MET minutes calculator tracks activity dose.
For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
What is predicted body weight?
A height-and-sex-based estimate of lean mass used for drug and ventilator dosing: 50 + 0.91 x (cm - 152.4) for men, 45.5 + 0.91 x (cm - 152.4) for women. It approximates the body size the lungs were built for, unlike actual weight.
Why not use actual body weight for tidal volume?
Because lungs do not grow with obesity. Dosing breaths by actual weight in heavy patients delivers excessive volumes that injure lungs. Predicted weight keeps the breath matched to lung size.
What is lung-protective ventilation?
Ventilating with 6-8 mL/kg predicted body weight and plateau pressures under 30 cm H2O to avoid ventilator-induced lung injury. The ARDSNet trial proved 6 mL/kg saves lives versus 12 mL/kg in ARDS.
What tidal volume should I use for ARDS?
6 mL/kg of predicted body weight, per ARDSNet. Some protocols allow up to 8 if needed for comfort or synchrony, but 6 is the evidence-based starting point.
Does height really determine lung size?
Closely enough for dosing. Lung volumes correlate strongly with height and sex, which is why the equations use only those two inputs. It is an estimate, but a well-validated one.
What is plateau pressure?
The pressure in the small airways at end-inspiration with no flow, reflecting alveolar distension. Lung-protective strategy keeps it at or below 30 cm H2O alongside low tidal volumes.
Can tidal volume be too low?
Excessively low volumes risk atelectasis and severe acidosis from inadequate ventilation. The 6 mL/kg target balances injury prevention against gas exchange, often tolerating mild permissive hypercapnia.
Do children use the same formula?
No. Pediatric predicted weights and tidal volume targets differ by age and size, and neonatal ventilation is its own specialty. This calculator is for adults.
What if the patient is very short or tall?
The equations extrapolate less reliably at extremes. Clinical judgment adjusts, and plateau pressure becomes the more important safety gauge when the weight estimate is uncertain.
Who sets ventilator tidal volumes?
ICU physicians and respiratory therapists, titrated to blood gases, pressures, and patient comfort. This calculator reproduces the standard arithmetic for education; it never sets a real ventilator.