SpO₂/FiO₂ Ratio Calculator — Non-Invasive P/F Ratio Surrogate

The SpO₂/FiO₂ (S/F) ratio is a validated non-invasive surrogate for the PaO₂/FiO₂ (P/F ratio) — the gold-standard oxygenation measurement requiring arterial blood gas (ABG). The S/F ratio uses pulse oximetry SpO₂ and the fraction of inspired oxygen (FiO₂) to estimate oxygenation status. Rice et al. (2007) validated the S/F ratio against P/F ratio in ARDS patients: S/F < 235 correlates with P/F < 200 (moderate ARDS); S/F < 315 correlates with P/F < 300 (mild ARDS). The ROX Index (SpO₂/FiO₂ ÷ RR) is derived from the S/F ratio and used specifically to predict HFNC success vs. failure.