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SpO₂/FiO₂ (S/F) Ratio

The SpO₂/FiO₂ ratio is a non-invasive surrogate for PaO₂/FiO₂ when SpO₂ is ≤97%. In Rice 2007, S/F = 64 + 0.84 × (P/F); an S/F of 235 corresponds to a P/F of 200 and 315 to a P/F of 300.

SpO₂ (%)
FiO₂ (% or fraction) (%)

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Interpretation

Result	Meaning
< 235	≈ P/F < 200
235–314	≈ P/F 200–300
≥ 315	≈ P/F > 300

Formula

S/F = SpO₂ (%) ÷ FiO₂ (fraction). Rice 2007: S/F = 64 + 0.84 × (P/F), so P/F ≈ (S/F − 64) ÷ 0.84.

Clinical pearls

Only valid when SpO₂ ≤97%; titrate oxygen down or use an arterial gas if saturation is higher.
The 2023 global ARDS definition accepts SpO₂/FiO₂ ≤315 (with SpO₂ ≤97%) as an oxygenation criterion; SOFA-2 also allows it when PaO₂ is unavailable.
Pulse oximetry overestimates saturation in patients with darker skin pigmentation, which can mask hypoxemia.

Examples

sf ratio spo2 95 fio2 40%S/F ratio for sats of 92% on FiO2 0.6 spo2 88 on 100% oxygen sf ratio

FAQ

What S/F ratio corresponds to a P/F of 300?

An S/F ratio of 315 corresponds to a P/F of 300, and 235 corresponds to a P/F of 200 (Rice 2007).

When can I use S/F instead of P/F?

When no arterial blood gas is available and SpO₂ is 97% or less.

Evidence

Rice TW et al. Comparison of the SpO2/FIO2 ratio and the PaO2/FIO2 ratio in patients with acute lung injury or ARDS. Chest 2007;132:410–7.

Pulmonary & Critical Care · Reviewed 2026-10-14 · Methodology