Airway Resistance (Raw) Calculator
Isolate the resistive load on the ventilated lung — Raw = (PIP − Pplat) ÷ flow — separating an airway problem (bronchospasm, secretions, a narrowed tube) from a stiff-lung problem, with the L/min → L/s conversion shown.
Written by Apex Respiratory Editorial Team
Read during an inspiratory pause (flow = 0).
Constant (square-wave) flow. 60 L/min is the standard reference.
Enter PIP, plateau pressure, and inspiratory flow to calculate airway resistance.
Reading airway resistance
PIP − Pplat is the purely resistive pressure drop; dividing by the inspiratory flow (converted to L/s) gives resistance in cmH₂O/L/s. The flow trap: ventilators read flow in L/min, so it must be divided by 60 first — at the standard 60 L/min this equals 1 L/s and Raw simply equals PIP − Pplat, but a 30 L/min flow doubles the computed Raw.
Normal for an intubated, ventilated adult is roughly 5–15 cmH₂O/L/s at 60 L/min; the endotracheal tube itself contributes much of that (native-airway normal is only ~0.5–2.5).
Valid only with a constant (square-wave) inspiratory flow and a true inspiratory pause — a decelerating flow pattern makes the simple formula invalid. The teaching contrast: a rising peak with a stable plateau is a resistance problem (Raw ↑); a rising peak with a rising plateau is a compliance problem.
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021.
- Hess DR, Kacmarek RM. Essentials of Mechanical Ventilation. 4th ed. McGraw-Hill Education; 2019.