SpO2/FiO2 Ratio Calculator

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

Use the SpO2/FiO2 Ratio Calculator

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

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

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

The SpO2/FiO2 (SF) ratio is a non-invasive surrogate for the PaO2/FiO2 (PF) ratio, used to assess the degree of hypoxemia and classify acute respiratory distress syndrome (ARDS) severity. Because SF relies on pulse oximetry rather than an arterial blood gas, it is practical in settings where invasive monitoring is unavailable or impractical. Published conversion formulas (Rice et al. 2007) allow clinicians to estimate the corresponding PF ratio from an SF ratio, enabling triage decisions consistent with the Berlin Definition of ARDS. An SF ratio below 315 suggests mild ARDS, below 235 moderate ARDS, and below 148 severe ARDS.

How it works

  1. Enter the SpO2 (peripheral oxygen saturation) as a percentage, typically from pulse oximetry.
  2. Enter the FiO2 (fraction of inspired oxygen) as a decimal (0.21–1.0) or percentage (21–100%).
  3. The calculator divides SpO2 by FiO2 to yield the SF ratio.
  4. The Rice et al. relationship estimates the equivalent PaO2/FiO2 (PF) ratio as PF = (SF − 64) / 0.84.
  5. ARDS severity is classified per the Berlin Definition thresholds mapped to SF equivalents.
  6. 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

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