Air-Entrainment Ratio & Total Flow Calculator
The device math behind air-entrainment ('Venturi') masks — the air-to-oxygen ratio that sets the FiO₂ and the total flow that determines whether the delivered FiO₂ is accurate, with the formula shown.
Written by Apex Respiratory Editorial Team
Enter as a percent (40) or a fraction (0.4).
The flowmeter setting feeding the jet.
Enter the set FiO₂ and O₂ flow to calculate the air-entrainment ratio and total flow.
Reading the entrainment ratio
An air-entrainment device drives a jet of 100% oxygen past a port that entrains room air. The air-to-oxygen ratio, (100 − FiO₂%) ÷ (FiO₂% − 21), sets the delivered FiO₂; total flow = (air parts + 1) × the set O₂ flow. The 21 is the oxygen percent of the entrained room air.
Commonly taught ratios (which the exact formula rounds to): 24% → 25:1 · 28% → 10:1 · 31% → 7:1 · 35% → 5:1 · 40% → 3:1 · 50% → 1.7:1 · 60% → 1:1.
As the set FiO₂ rises, the entrainment ratio and total flow both fall. A 24% device floods the patient with 100+ L/min (a true high-flow, fixed-performance system), while a 60% device barely exceeds its set O₂ flow. When total flow drops below inspiratory demand (~30–40 L/min, or roughly 3× minute ventilation), the patient entrains extra room air around the device and the delivered FiO₂ falls below the set value.
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.
- Cairo JM. Mosby's Respiratory Care Equipment. 11th ed. Elsevier; 2022.