Quick answer: Male PBW = 50 + 2.3 × (height in inches − 60). Female PBW = 45.5 + 2.3 × (height in inches − 60). In metric, replace the bracket with 0.91 × (height in cm − 152.4). A 70-inch man has a PBW of 73 kg, giving an ARDSNet tidal volume of 438 mL at 6 mL/kg. Height and sex only — actual weight never enters it.
Why height, and only height
This is the part worth understanding before the arithmetic, because it is what the whole protocol rests on.
Lung volume tracks height and sex. It does not track body mass. Two men who are both 5′10″ have close to the same lung capacity whether one weighs 70 kg or 130 kg — the extra 60 kg is adipose tissue and it does not come with extra alveoli. In fact obesity reduces functional residual capacity by pressing on the diaphragm, so the heavier patient has less usable lung, not more.
Now set a ventilator from actual weight. At 8 mL/kg, the 70 kg man receives 560 mL and the 130 kg man receives 1,040 mL — into lungs of the same size. That is roughly 14 mL/kg of real lung, deep into the range the ARDSNet trial showed increases mortality. The predicted body weight equation exists precisely to break that link.
| Two men, both 5′10″ (70 in) | Patient A | Patient B |
|---|---|---|
| Actual body weight | 70 kg | 130 kg |
| Predicted body weight | 73.0 kg | 73.0 kg |
| Lung size | Same | Same |
| VT if set at 6 mL/kg actual | 420 mL | 780 mL |
| VT at 6 mL/kg PBW | 438 mL | 438 mL |
| Error from using actual weight | −4% | +78% |
That +78% is the entire clinical argument in one number. Note also that it runs the other way for underweight patients: a cachectic patient set from actual weight is under-ventilated. PBW protects both directions.
PBW formula in centimetres and inches
Male: PBW (kg) = 50 + 2.3 × (heightin − 60)
Female: PBW (kg) = 45.5 + 2.3 × (heightin − 60)
Metric
Male: PBW (kg) = 50 + 0.91 × (heightcm − 152.4)
Female: PBW (kg) = 45.5 + 0.91 × (heightcm − 152.4)
Both forms describe the same line. 60 inches is 152.4 cm — the anchor point — and 2.3 kg per inch is 0.9055 kg per centimetre, conventionally rounded to 0.91. The only structural difference between the sexes is the intercept: 50 kg versus 45.5 kg at 5 feet. The slope is identical.
A footnote that resolves a common confusion. Because 0.91 is a rounded 0.9055, the metric and imperial equations do not return bit-identical answers. For a 178 cm patient the imperial route gives 73.18 kg and the metric route 73.30 kg — a difference of 0.12 kg, which is about 0.7 mL of tidal volume. It is clinically meaningless, but it is why two calculators, two textbooks or two colleagues can disagree in the first decimal place and both be right. This calculator shows you both.
ARDSNet tidal volume chart by height
Predicted body weight and the 6 mL/kg starting tidal volume, by height and sex. The 4 and 8 mL/kg columns are the protocol's lower and upper bounds.
| Height | Male PBW | VT 6 mL/kg | Female PBW | VT 6 mL/kg |
|---|---|---|---|---|
| 4′10″ / 147 cm | 45.4 kg | 272 mL | 40.9 kg | 245 mL |
| 5′0″ / 152 cm | 50.0 kg | 300 mL | 45.5 kg | 273 mL |
| 5′2″ / 157 cm | 54.6 kg | 328 mL | 50.1 kg | 301 mL |
| 5′4″ / 163 cm | 59.2 kg | 355 mL | 54.7 kg | 328 mL |
| 5′6″ / 168 cm | 63.8 kg | 383 mL | 59.3 kg | 356 mL |
| 5′8″ / 173 cm | 68.4 kg | 410 mL | 63.9 kg | 383 mL |
| 5′10″ / 178 cm | 73.0 kg | 438 mL | 68.5 kg | 411 mL |
| 6′0″ / 183 cm | 77.6 kg | 466 mL | 73.1 kg | 439 mL |
| 6′2″ / 188 cm | 82.2 kg | 493 mL | 77.7 kg | 466 mL |
| 6′4″ / 193 cm | 86.8 kg | 521 mL | 82.3 kg | 494 mL |
| 6′6″ / 198 cm | 91.4 kg | 548 mL | 86.9 kg | 521 mL |
Two things stand out. First, the range is narrow — every adult in that table falls between roughly 270 and 550 mL at 6 mL/kg. If you have set a tidal volume outside that window, check the height entry. Second, the male and female rows differ by a constant 4.5 kg, about 27 mL, across the whole table.
Where PBW sits in the ARDSNet protocol
Predicted body weight is step one of a sequence. The rest of it matters because tidal volume is not set once and left alone:
- Calculate PBW from height and sex.
- Set VT at 6 mL/kg PBW as the initial target. Some protocols start at 8 mL/kg and reduce over the first few hours.
- Measure plateau pressure with a 0.5-second inspiratory hold, at least every four hours and after every change.
- If Pplat > 30 cmH₂O, reduce VT in 1 mL/kg steps — to 5, then 4 mL/kg as the floor.
- If Pplat < 25 cmH₂O and VT is below 6 mL/kg, increase in 1 mL/kg steps back toward 6.
- If there is severe breath stacking or pH < 7.15, VT may go up in 1 mL/kg steps to a maximum of 8 mL/kg.
| Parameter | Target | Action if outside |
|---|---|---|
| Tidal volume | 6 mL/kg PBW (range 4–8) | Adjust in 1 mL/kg steps |
| Plateau pressure | ≤ 30 cmH₂O | Reduce VT by 1 mL/kg |
| Arterial pH | 7.30–7.45 | Raise rate to 35, then consider bicarbonate |
| Respiratory rate | Up to 35 breaths/min | Watch for breath stacking and auto-PEEP |
| Oxygenation | PaO₂ 55–80 mmHg or SpO₂ 88–95% | Move along the PEEP/FiO₂ table |
The pH row is the trade the protocol makes explicit. Smaller breaths mean less CO₂ cleared per breath, so permissive hypercapnia is expected and accepted — the respiratory rate goes up to compensate, and a degree of acidosis is tolerated rather than abandoning the low tidal volume. Once VT is set, the tidal volume calculator covers minute ventilation and the rate side of that balance.
PBW versus the other ideal body weight formulas
Several IBW equations are in circulation and they do not agree. ARDSNet uses Devine; substituting another one changes the ventilator setting.
| Formula | Male equation | Male, 70″ | Female, 64″ | Used for |
|---|---|---|---|---|
| Devine / ARDSNet | 50 + 2.3 per inch > 60″ | 73.0 kg | 54.7 kg | Ventilator tidal volume |
| Robinson (1983) | 52 + 1.9 per inch > 60″ | 71.0 kg | 55.8 kg | Nutrition, some drug dosing |
| Miller (1983) | 56.2 + 1.41 per inch > 60″ | 70.3 kg | 58.5 kg | Research comparison |
| Hamwi (1964) | 48 + 2.7 per inch > 60″ | 75.0 kg | 54.3 kg | Dietetics |
| BMI 22 method | 22 × height(m)² | 69.3 kg | 57.9 kg | Population estimates |
The spread for that 70-inch man is 69.3 to 75.0 kg — about 8%, or 34 mL of tidal volume at 6 mL/kg. Not catastrophic, but enough to matter in a trial and enough to explain discrepancies between two units' calculators. For ventilator settings, use Devine/ARDSNet. The others exist for nutrition and pharmacokinetics and were never validated against lung volume.
Common mistakes & pro tips
- Using actual body weight. The error this protocol exists to prevent. In a patient with obesity it can nearly double the delivered volume.
- Estimating height by eye. Height is the only variable, so an error in it flows straight through. A 2-inch overestimate adds 4.6 kg of PBW and 28 mL of tidal volume. Measure supine, or use knee height or ulna length if the patient cannot be straightened.
- Applying the male equation to a female patient. A flat 4.5 kg error, about 27 mL.
- Mixing formula families. Robinson or Hamwi IBW is not interchangeable with ARDSNet PBW for ventilation.
- Setting VT and never rechecking Pplat. Compliance changes through the illness; 6 mL/kg is a starting point, not a fixed prescription.
- Pro tip: sanity-check the range. Almost every adult lands between 270 and 550 mL at 6 mL/kg. Outside that, re-enter the height.
- Pro tip: write PBW on the ventilator. Putting the PBW and the 4/6/8 mL/kg volumes on a label by the machine means the next clinician adjusting settings does not have to recalculate or guess.
How to use this calculator
- Select sex — this sets the intercept, 50 kg for male and 45.5 kg for female.
- Enter height in centimetres, inches, or feet and inches. The calculator shows both the metric and imperial results so you can see the 0.1 kg convention difference.
- Read the tidal volume ladder — 4, 6, 7 and 8 mL/kg are all shown, since the protocol moves between them.
- Optionally enter actual body weight to see BMI, adjusted body weight, and how far off an actual-weight-based tidal volume would have been.
Frequently asked questions
What is the ARDSNet predicted body weight formula?
For men, PBW = 50 + 2.3 × (height in inches − 60). For women, PBW = 45.5 + 2.3 × (height in inches − 60). The metric versions are PBW = 50 + 0.91 × (height in cm − 152.4) for men and 45.5 + 0.91 × (height in cm − 152.4) for women. Only height and sex enter the calculation — actual body weight is deliberately excluded.
Why use predicted body weight instead of actual weight for tidal volume?
Because lung size is determined by height and sex, not by body fat. A 120 kg patient and a 70 kg patient of the same height have essentially the same lung volume. Setting tidal volume from actual weight would deliver almost twice the intended volume to the heavier patient's identical lungs, causing the volutrauma that lung-protective ventilation exists to prevent. This is the single most important idea in the ARDSNet protocol.
What is the tidal volume target in ARDS?
Start at 6 mL per kg of predicted body weight and keep plateau pressure at or below 30 cmH₂O. If plateau pressure exceeds 30, reduce tidal volume in 1 mL/kg steps down to a minimum of 4 mL/kg. If the patient has severe breath stacking or a pH below 7.15, tidal volume may be raised in 1 mL/kg steps to a maximum of 8 mL/kg. The 2000 ARDSNet trial found lower mortality at 6 mL/kg compared with 12 mL/kg.
Is predicted body weight (PBW) the same as ideal body weight (IBW)?
In practice, yes — the ARDSNet predicted body weight equation is numerically identical to the 1974 Devine ideal body weight formula. The critical care literature prefers the term predicted body weight to avoid implying that the number is a weight the patient ought to weigh. It is a proxy for lung size, not a nutritional or aesthetic target. Other ideal body weight formulas such as Robinson, Miller and Hamwi give different numbers and should not be substituted for ventilator settings.
Do the imperial and metric formulas give the same answer?
Almost, but not exactly. The imperial coefficient of 2.3 kg per inch converts to 0.9055 kg per centimetre, but the metric form is conventionally rounded to 0.91. For a 178 cm patient the two versions differ by about 0.12 kg — roughly 0.7 mL of tidal volume at 6 mL/kg. Clinically irrelevant, but it explains why two calculators can disagree in the first decimal place.
What is adjusted body weight and when is it used?
Adjusted body weight = IBW + 0.4 × (actual body weight − IBW). It is used for dosing certain drugs in obesity, where the drug distributes partly into adipose tissue — aminoglycosides are the classic example. It is not used for tidal volume: ventilator settings use predicted body weight alone, with no adjustment for actual weight, because lungs do not grow with body fat.
What does PBW mean in medicine?
PBW stands for predicted body weight. Clinicians estimate it from height and sex alone and use it to size ventilator tidal volume, because lung size tracks height rather than body fat. The ARDSNet equations are Male PBW = 50 + 0.91 × (height in cm − 152.4) and Female PBW = 45.5 + 0.91 × (height in cm − 152.4). PBW is not the measured weight: a 175 cm man has a PBW of about 70.6 kg whether he weighs 60 kg or 120 kg.
References & further reading
- The Acute Respiratory Distress Syndrome Network (2000), "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 342:1301–1308 — the ARMA trial.
- Devine, B. J. (1974), "Gentamicin therapy", Drug Intelligence and Clinical Pharmacy — the origin of the 50/45.5 + 2.3 per inch equation.
- NIH NHLBI ARDS Network ventilator protocol card — the full VT, plateau pressure and PEEP/FiO₂ algorithm.
- Linares-Perdomo O, East TD, Brower R, Morris AH. Standardizing predicted body weight equations for mechanical ventilation tidal volume settings. Chest 2015;148(1):73–78. doi:10.1378/chest.14-2843. Compares Devine with the Robinson, Miller and Hamwi formulas.
Educational use only. This calculator reproduces a published equation and protocol targets. It is not a substitute for clinical judgement, local protocol, or bedside assessment, and it must not be used as the sole basis for a ventilator setting. See our accuracy policy.