S/F Ratio Calculator
The noninvasive oxygenation surrogate — SpO₂ ÷ FiO₂ — that grades ARDS severity from a pulse-oximeter reading when no arterial blood gas is available, with the 2023 global-definition thresholds and the SpO₂ ≤ 97% reliability rule.
Written by Apex Respiratory Editorial Team
Most reliable at or below 97%.
Enter as a fraction (0.5) or a percent (50).
Enter SpO₂ and FiO₂ to calculate the S/F ratio.
Reading the S/F ratio
The S/F ratio (SpO₂ ÷ FiO₂) estimates the P/F ratio without an arterial stick. From Rice et al., an S/F of 315 corresponds to a P/F of about 300 (the mild-ARDS cutoff) and an S/F of 235 to a P/F of about 200 (moderate). The 2023 global definition of ARDS formally adopted S/F ≤ 315 as a noninvasive oxygenation criterion.
Reliable only while SpO₂ ≤ 97%. Above that, the reading sits on the flat top of the oxyhemoglobin dissociation curve, where one SpO₂ can map to a wide range of PaO₂ — so a reassuring S/F can hide a much worse true P/F. Titrate FiO₂ down to a target SpO₂ of ~94–96% before trusting the number.
S/F now sits alongside the P/F ratio, the A–a gradient, VD/VT, and the oxygenation index as a core oxygenation calculation RTs are expected to know — a board-relevant shift driven by the 2023 global definition. It grades oxygenation only: ARDS also requires qualifying respiratory support, bilateral opacities, and a non-cardiogenic cause.
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
- Rice TW, Wheeler AP, Bernard GR, et al. Comparison of the SpO2/FiO2 ratio and the PaO2/FiO2 ratio in patients with acute lung injury or ARDS. Chest. 2007;132(2):410-417.
- Matthay MA, Arabi Y, Arroliga AC, et al. A New Global Definition of Acute Respiratory Distress Syndrome. Am J Respir Crit Care Med. 2024;209(1):37-47.
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021.