What is the SF ratio?

The SF ratio is SpO2 divided by FiO2. It is a non-invasive estimate of respiratory function that parallels the arterial PaO2/FiO2 (PF) ratio. Because pulse oximetry is widely available, SF is practical in settings where arterial blood gas measurement is not feasible.

How does SF compare to PF ratio?

Rice et al. (2007) derived conversion equations between SF and PF. The SF ratio correlates well with PF when SpO2 is ≤ 97%, because at higher saturations the oxyhemoglobin dissociation curve is flat and SpO2 no longer accurately reflects PaO2. For reliable estimation, SpO2 should be 80–97%.

What SF ratio values indicate ARDS?

Based on Berlin Definition PF equivalents: SF < 315 corresponds to mild ARDS (PF 200–300), SF < 235 to moderate ARDS (PF 100–200), and SF < 148 to severe ARDS (PF < 100). These thresholds assume PEEP ≥ 5 cmH2O in the original ARDS criteria.

What FiO2 should I enter for room air?

Room air has an FiO2 of approximately 0.21 (21%). Enter 0.21 or 21 in the input field. If the patient is on supplemental oxygen, calculate FiO2 based on the delivery device and flow rate (e.g. nasal cannula at 2 L/min ≈ 0.28).

Is this tool a substitute for arterial blood gas?

No. The SF ratio is a non-invasive estimate with important limitations, especially when SpO2 > 97% or when hemoglobin abnormalities affect pulse oximetry accuracy. Clinical decisions should be made by qualified clinicians using all available data. This calculator is for educational purposes only.

SpO2/FiO2 Ratio Calculator

Name: SpO2/FiO2 Ratio Calculator
Availability: InStock
Author: Nham Vu

Enter SpO2 and FiO2 values to calculate the SF ratio, estimate the equivalent PF ratio, and classify ARDS severity.

For educational use only. Not a substitute for clinical judgment or arterial blood gas analysis. All results must be interpreted by qualified clinicians.

Patient Values

SpO₂ — Oxygen Saturation (%)

Normal range: 95–100%. Best accuracy at 80–97%.

FiO₂ — Fraction of Inspired Oxygen

Enter as decimal (0.21–1.0) or percent (21–100). Room air = 0.21.

Common FiO₂ Values

Room air0.21

Nasal cannula 2 L/min~0.28

Nasal cannula 4 L/min~0.36

Simple face mask 8 L/min~0.50

NRB mask 12 L/min~0.80

100% O₂ (vent)1.00

Enter SpO₂ and FiO₂ values,
then click Calculate.

ARDS Severity Classification (Berlin Definition + SF Equivalents)

Category	PF Ratio (mmHg)	SF Ratio (equiv.)	Mortality (est.)
No ARDS	> 300	> 315	—
Mild ARDS	201–300	235–315	~27%
Moderate ARDS	101–200	148–234	~32%
Severe ARDS	≤ 100	< 148	~45%

SF thresholds derived from Rice et al. (2007). PF thresholds from the 2012 Berlin Definition of ARDS. PEEP ≥ 5 cmH2O assumed for formal ARDS criteria.

Summary

Enter SpO2 and FiO2 values to calculate the SF ratio, estimate the equivalent PF ratio, and classify ARDS severity.

How it works

Enter the SpO2 (peripheral oxygen saturation) as a percentage, typically from pulse oximetry.
Enter the FiO2 (fraction of inspired oxygen) as a decimal (0.21–1.0) or percentage (21–100%).
The calculator divides SpO2 by FiO2 to yield the SF ratio.
The Rice et al. conversion formula estimates the equivalent PaO2/FiO2 (PF) ratio: PF = 64 × (SF / 100) if SF ≤ 235, or PF = (SF − 64) if SF > 235.
ARDS severity is classified per the Berlin Definition thresholds mapped to SF equivalents.
Results include the SF ratio, estimated PF ratio, ARDS category, and clinical interpretation.

Use cases

Screen for ARDS when arterial blood gas is unavailable or impractical.
Rapidly classify respiratory failure severity in resource-limited or field settings.
Monitor trend in oxygenation without repeated arterial blood sampling.
Validate oxygenation status in non-intubated patients on supplemental oxygen.
Use in COVID-19 and other viral pneumonias where early ARDS recognition is critical.
Support triage decisions during mass-casualty events.
Calculate SF ratio for research documentation or case reporting.
Cross-check clinical impression with an objective oxygenation index.

Frequently Asked Questions

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Last updated: 2026-05-23 · Reviewed by Nham Vu