SpO₂/FiO₂ (S/F) Ratio

Introduction

The S/F ratio calculator estimates how well a patient is oxygenating using only a pulse oximeter reading and the oxygen being delivered, with no arterial blood gas required. It serves as a noninvasive substitute for the PaO₂/FiO₂ (P/F) ratio, the standard measure of oxygenation used to diagnose and grade acute respiratory distress syndrome (ARDS) and acute lung injury.

Where it’s used

The ratio is particularly useful in resource-limited settings where arterial blood gases aren’t readily available. It also suits patients for whom repeated arterial sampling is impractical, such as children and patients outside the ICU. It allows continuous trending, because pulse oximetry is always running. It appears in related scores too. The ROX index, for instance, divides the S/F ratio by respiratory rate to help predict whether high-flow nasal oxygen will fail.

How it works

The calculation is simple:

S/F Ratio = (SpO2 / FiO2) × 100

The tool needs two inputs. SpO₂ is the oxygen saturation from pulse oximetry, entered as a percentage. FiO₂ is the fraction of inspired oxygen: 0.21 on room air, or the set value on a ventilator, high-flow device, or mask. For example, a patient with an SpO₂ of 92% on an FiO₂ of 0.40 has an S/F ratio of 92 ÷ 0.40 = 230.

Many versions of the calculator also convert the result into an estimated P/F ratio. The most widely cited conversion comes from Rice and colleagues (2007), who found a roughly linear relationship in ARDS patients: S/F ≈ 64 + 0.84 × P/F.

Interpretation & Guidance

Lower ratios mean worse oxygenation. The S/F ratio has been formally adopted in some ARDS definitions.

Reference (Rice, 2007) for adult interpretation.

• An S/F ratio of 315 approximates a P/F ratio of 300.

• An S/F ratio of 235 correlates with a P/F ratio of 200.

Factors that may impact accuracy due to effect on pulse oximetry, include methemoglobinemia, carboxyhemoglobinemia, poor peripheral perfusion, skin pigmentation, nail polish, and movement artifacts.

Considerations for use in ARDS

The S/F ratio is just one of several criteria used in the diagnosis and severity classification of ARDS. Refer to the “global definition” of ARDS (Matthay et al., 2024).

S/F Ratio Thresholds for Specific ARDS Categories.

*If SpO2 ≤97%
**Neither positive end-expiratory pressure nor a minimum flow rate of oxygen is required for diagnosis in resource-limited settings.

Limitations

The most important caveat is that the S/F ratio is only reliable when SpO₂ is 97% or lower. Above that level, the oxyhemoglobin dissociation curve flattens, so large changes in PaO₂ produce almost no change in SpO₂. Clinicians often lower the FiO₂ until saturation falls to 97% or below before calculating.

Other limitations include:

• Inaccurate oximeter readings: Readings can be off with poor perfusion, motion, carbon monoxide poisoning, methemoglobinemia, and darker skin pigmentation, where oximeters tend to overestimate saturation and can mask hypoxemia.

• Imprecise FiO₂ estimates: On low-flow devices such as nasal cannula, delivered FiO₂ is only an estimate, commonly approximated as about 21% plus 3–4% per liter of flow, which introduces error.

• Not a full replacement for a blood gas: It’s an approximation of the P/F ratio and gives no information about CO₂ or acid–base status.

Rice TW, Wheeler AP, Bernard GR, Hayden DL, Schoenfeld DA, Ware LB; National Institutes of Health, National Heart, Lung, and Blood Institute ARDS Network. Comparison of the SpO2/FIO2 ratio and the PaO2/FIO2 ratio in patients with acute lung injury or ARDS. Chest. 2007 Aug;132(2):410-7.