Respiratory Physiology Calculator
SpO2/FiO2 Ratio Calculator
Calculate the S/F ratio from pulse oximetry and FiO2 as a non-invasive surrogate for PaO2/FiO2, with optional ABG comparison and ARDS severity context.
Quick Answer
The SpO2/FiO2 ratio (S/F) divides pulse oximetry saturation by inspired oxygen fraction as a non-invasive surrogate for the PaO2/FiO2 (P/F) ratio when arterial blood gas is unavailable. Under the 2023 global ARDS definition, S/F ≤315 (when SpO2 ≤97%) supports ARDS-range hypoxemia. This calculator applies S/F = SpO2 (%) / FiO2 with optional P/F comparison and Berlin severity bands — for clinical education, not diagnosis.
Calculate SpO2/FiO2 Ratio
Enter SpO2 and FiO2 to compute S/F. Add PaO2 optionally to compare with the P/F ratio.
SpO2/FiO2 Ratio (S/F)
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What Is the SpO2/FiO2 Ratio (S/F Ratio)?
The SpO2/FiO2 ratio divides transcutaneous oxygen saturation by the fraction of inspired oxygen. Clinicians use it to estimate oxygenation efficiency without an arterial blood gas—especially during acute respiratory failure, high-flow nasal cannula therapy, and emergency or resource-limited care.
Because pulse oximetry is continuous and non-invasive, S/F supports trending and screening. It does not replace full gas exchange assessment when management decisions depend on precise arterial values.
SpO2/FiO2 vs PaO2/FiO2 (P/F Ratio)
The PaO2/FiO2 ratio (Horowitz index) is the traditional oxygenation metric in Berlin ARDS criteria. The S/F ratio approximates P/F when ABG is unavailable. Rice and colleagues reported S/F ≈315 maps to P/F ≈300, and S/F ≈235 to P/F ≈200.
When PaO2 is available, compare both indices. Disagreement can occur with high SpO2, poor perfusion, or device-related FiO2 uncertainty. Obtain ABG when uncertainty would change diagnosis, severity classification, or ventilator strategy.
S/F Ratio and ARDS Berlin Criteria
Berlin ARDS severity uses PaO2/FiO2 with minimum PEEP or CPAP ≥5 cm H2O: mild (>200 to ≤300 mmHg), moderate (>100 to ≤200), severe (≤100). Berlin criteria require bilateral opacities, timing within one week, and exclusion of cardiogenic edema—not oxygenation alone.
S/F is a surrogate when ABG is not obtained. It should not be applied as a standalone Berlin diagnostic rule without imaging, clinical context, and appropriate ventilatory support documentation.
2023 Global ARDS Definition and S/F Thresholds
The 2023 global ARDS definition formally accepts SpO2/FiO2 when SpO2 ≤97%, using Rice linear cutoffs aligned with P/F tiers:
- Mild: >235 to ≤315 (approximates P/F >200 to ≤300)
- Moderate: >148 to ≤235 (approximates P/F >100 to ≤200)
- Severe: ≤148 (approximates P/F ≤100)
Non-intubated ARDS oxygenation may be assessed with S/F ≤315 on HFNC ≥30 L/min or NIV/CPAP with ≥5 cm H2O PEEP, when SpO2 ≤97%. Resource-limited modified criteria may apply S/F ≤315 without PEEP or minimum flow requirements.
When to Use S/F Ratio Instead of Arterial Blood Gas
SpO2/FiO2 Ratio in COVID-19 and Acute Respiratory Failure
During COVID-19 surges, many centers relied on S/F for triage, proning decisions, and ICU transfer thresholds when ABG sampling was limited. S/F remains useful for monitoring response to HFNC, CPAP, and ventilator adjustments, provided SpO2 is ≤97% and perfusion is adequate.
Silent hypoxemia and device-related FiO2 variability can mask severity. Combine S/F with respiratory rate, work of breathing, imaging, lactate, and hemodynamics—not oximetry alone.
Pediatric Oxygenation: S/F vs Oxygenation Index
Pediatric ARDS (PARDS) under PALICC stratifies invasively ventilated children with oxygenation index (OI) or oxygen saturation index (OSI), which include mean airway pressure. S/F and P/F may still apply with non-invasive support.
OSI = FiO2 × MAP × 100 / SpO2 is a ventilator-burden metric—not interchangeable with simple S/F. Use the oxygenation index calculator when MAP and invasive ventilation data are available.
Limitations of Pulse Oximetry for S/F Assessment
SpO2 accuracy falls at extremes of perfusion, motion, and the oxyhemoglobin dissociation curve. Skin pigmentation, nail polish, venous pulsation, and carboxyhemoglobin/methemoglobin can bias readings. At SpO2 >97%, S/F poorly estimates P/F because small PaO2 changes produce minimal saturation change.
Recent studies highlight disagreement between SpO2-based and ABG-based severity bands in ARDS and PARDS. Treat S/F as a practical surrogate with known limits—not a definitive classifier without corroborating data.
Worked Example
SpO2 88%, FiO2 0.60, optional PaO2 70 mmHg.
S/F = 88 / 0.60 = 147 → severe range by 2023 global S/F thresholds (≤148).
P/F = 70 / 0.60 = 117 mmHg → moderate range by Berlin PaO2/FiO2 tiers (>100 to ≤200).
Sources and Further Reading
- A New Global Definition of Acute Respiratory Distress Syndrome (2023)
- Rice TW et al. Comparison of the SpO2/FiO2 ratio and PaO2/FiO2 ratio (2007)
- StatPearls: Acute Respiratory Distress Syndrome
- Limitations of SpO2-based oxygenation indices in ARDS and PARDS (2025)
- Competitive landscape: MDCalc SpO₂/FiO₂ Ratio is a trusted ICU tool with 2023 global ARDS S/F thresholds but accepts FiO₂ as percent only, lacks side-by-side P/F comparison, and does not link to oxygenation index or pediatric PARDS context. Medaptly P/F Ratio Calculator is ABG-first with optional S/F estimate and Berlin PEEP context — not an S/F-first workflow with Rice linear cutoffs, 2023 global severity bands, and COVID/resource-limited guidance. NovaPharmaNews provides a free S/F calculator with optional P/F comparison, Berlin/global thresholds, and oxygenation index cross-links — no login required.