Respiratory

SpO₂/FiO₂ Ratio Calculator

SpO₂ divided by FiO₂, a stand-in for the P/F ratio when there is no blood gas; valid only with SpO₂ 97% or less.

Reviewed by Alex G, BSN, RN, CEN, CCRN, CMC, NREMT-P · From the Reference tab of the app

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Formula

SpO₂/FiO₂ = SpO₂ / (FiO₂ as decimal)

How it is used

Interpreting results for adults (Rice, 2007)

• 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.

S/F Ratio Thresholds for Specific ARDS Categories

• Nonintubated ARDS: ≤315 on HFNC (≥30 L/min) or NIV/CPAP (≥5 cm H₂O PEEP)
• Intubated ARDS (Mild): >235 and ≤315
• Intubated ARDS (Moderate): >148 and ≤235
• Intubated ARDS (Severe): ≤148
• Resource-limited settings: ≤315 (No PEEP or minimum flow required)

*Criteria applies if SpO₂ ≤ 97%

*S/F ratio is just one of several criteria used in the diagnosis and severity classification of ARDS.

Same formula as the app. Open it in the web app.

Related one-pagers

• Ventilator Modes and Settings
• ABG Interpretation in 6 Steps

Related calculators

• A-a Gradient
• Alveolar Gas Equation (PAO₂)
• Arterial Oxygen Content (CaO₂)
• Minute Ventilation
• P/F Ratio (PaO₂/FiO₂)
• Predicted Body Weight (PBW)
• Ventilator Rate for a Target PaCO₂

For education and exam preparation only. Follow your institution’s protocols. For education and exam preparation only. Verify results independently and follow your local protocols and medical direction. Not a substitute for clinical judgment.