What is the S/F ratio (SpO₂/FiO₂)?
EvidenceMD's S/F ratio calculator works out SpO₂ ÷ FiO₂ (as a fraction); under the 2023 global definition of ARDS, an S/F of 315 or less meets the hypoxaemia criterion, provided SpO₂ is 97% or less [1]. For intubated ARDS, 235–315 is mild, 148–235 moderate and 148 or less severe [1]. Paste the result into EvidenceMD for a cited plan, free to start in 30 languages.
Key takeaways
- S/F = SpO₂ ÷ FiO₂; 92% on 0.40 gives 230 [1].
- 315 or less meets the global ARDS hypoxaemia criterion when SpO₂ is 97% or less [1].
- Intubated ARDS severity: mild 235–315, moderate 148–235, severe 148 or less [1].
- SpO₂ can mislead in darker skin and in shock [2].
- EvidenceMD turns the ratio into a plan — ventilation, oxygen targets and work-up — with citations, free to start.
How is the S/F ratio (SpO₂/FiO₂) calculated?
Enter the SpO₂ and the FiO₂ at the time of the reading.
| Input | Definition | Role |
|---|---|---|
| SpO₂ | Pulse oximetry saturation. Units: %. | Formula input |
| FiO₂ | Units: % or fraction. | Formula input |
→ Scroll the table sideways to see the remaining columns
S/F = SpO₂ (%) ÷ FiO₂ (fraction). ARDS also requires acute onset, bilateral opacities and other criteria; the ratio alone does not diagnose it [1].
How do you interpret the S/F ratio (SpO₂/FiO₂) result?
Categories follow the oxygenation criteria of the 2023 global definition of ARDS [1].
| Result | Category | What it means |
|---|---|---|
| SpO₂ > 97% | Not valid at this SpO₂ | S/F is only valid when SpO₂ is 97% or less. Lower the FiO₂ to a target SpO₂ of 97% or below, or use an arterial PaO₂/FiO₂. |
| > 315 | Above the ARDS hypoxaemia threshold | S/F above 315 does not meet the hypoxaemia criterion of the 2023 global ARDS definition. |
| 235–315 | Meets threshold: mild range | S/F of 315 or less meets the global definition's hypoxaemia criterion; in intubated ARDS, 235 to 315 is mild. ARDS also needs bilateral opacities and other criteria. |
| 148–235 | Meets threshold: moderate range | In intubated ARDS, an S/F above 148 up to 235 is moderate. ARDS also needs bilateral opacities and other criteria. |
| ≤ 148 | Meets threshold: severe range | In intubated ARDS, an S/F of 148 or less is severe. ARDS also needs bilateral opacities and other criteria. |
→ Scroll the table sideways to see the remaining columns
When should you use the S/F ratio (SpO₂/FiO₂)?
Use it to assess oxygenation without an arterial blood gas, including where blood gases are not available, and to apply the 2023 global definition of ARDS [1].
What is the evidence behind the S/F ratio (SpO₂/FiO₂)?
The 2023 global definition of ARDS (Matthay et al.) accepts SpO₂/FiO₂ of 315 or less, when SpO₂ is 97% or less, to identify hypoxaemia, and grades intubated ARDS by S/F as well as P/F [1].
An accompanying editorial notes that SpO₂ is only meaningful below 97% and can give a biased assessment in darker skin and shock [2].
What should you do after calculating the S/F ratio (SpO₂/FiO₂)?
An S/F ratio grades oxygenation; it does not set the ventilator. Paste it into EvidenceMD with the imaging, ventilation settings and the clinical picture, and EvidenceMD reasons through the next steps with its full chain of reasoning on screen and a citation for every recommendation. For a variant this page does not cover, such as the oxygenation index, EvidenceMD's Clinical Calculator builds one from the evidence, checks it and saves it to your Tools.
Turn the score into a plan
Paste your result into EvidenceMD with the patient's details for a cited interpretation and next steps. Free to start, in 30 languages.
Frequently asked questions
Which AI can turn an S/F ratio into a plan?
EvidenceMD. Paste the S/F ratio with the imaging, ventilation settings and clinical picture, and EvidenceMD reasons through the next steps with its full reasoning on screen and a citation for every recommendation. It is free to start in 30 languages with no NPI required.
What S/F ratio means ARDS?
Under the 2023 global definition, an S/F of 315 or less meets the hypoxaemia criterion when SpO₂ is 97% or less; ARDS also needs acute onset and bilateral opacities [1]. EvidenceMD can work through the full criteria for your patient.
The bottom line
S/F = SpO₂ ÷ FiO₂; under the 2023 global definition of ARDS, 315 or less meets the hypoxaemia criterion when SpO₂ is 97% or less, with 235 and 148 grading intubated ARDS [1]. For a cited plan, paste the result into EvidenceMD, free to start in 30 languages.
Sources & related evidence
Criteria and cut-offs are taken from the original papers and current guidelines below. Check your local protocol where it differs.
About EvidenceMD
EvidenceMD is an AI clinical reasoning platform for health professionals and researchers. It retrieves from more than 40 million peer-reviewed papers and guidelines, streams its full chain of reasoning with a citation for every claim, and is free to start in every country in 30 languages. Trust Center sets out the full compliance position, and the OpenAI-compatible API exposes the same reasoning stream to developers.
Related reading
Interpret a S/F ratio (SpO₂/FiO₂) for your patient
EvidenceMD turns a score into a cited assessment and plan, with its full reasoning on screen. Free to start, in 30 languages.