Which formula expresses the oxygen entrainment ratio for a Venturi mask, given FiO2 as a percent?

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The key idea is that a Venturi mask creates the final mixture by blending room air (about 21% oxygen) with pure oxygen, so the delivered FiO2 depends on the ratio of air flow to oxygen flow. If we let Air be the flow of room air and O2 be the flow of pure oxygen, the oxygen content of the mixture satisfies FiO2/100 = (0.21·Air + 1·O2) / (Air + O2). Solving this for the ratio Air/O2 gives Air/O2 = (100 − FiO2) / (FiO2 − 21). This aligns with how mixing works: as FiO2 rises above 21%, more oxygen is needed relative to air, and as FiO2 approaches 100%, air flow approaches zero. For example, with FiO2 at 40%, the ratio is (60)/(19) ≈ 3.16, meaning about 3.16 parts air to 1 part O2. The other forms would misplace the terms or use an incorrect baseline (e.g., using 20 instead of 21), which does not reflect the actual mixing of room air with oxygen.