Why is the S/F ratio more useful than SpO2 alone?

SpO2 tells you the saturation of haemoglobin but not how much oxygen was required to achieve that saturation. An SpO2 of 95% on room air (FiO2 21%) indicates good gas exchange (S/F = 452). The same SpO2 of 95% on 60% FiO2 indicates severely impaired gas exchange (S/F = 158). The S/F ratio normalises SpO2 for the oxygen requirement, revealing the true efficiency of gas exchange. Illustrative example only -- verify with current clinical guidelines.

Is the S/F ratio the same as the P/F ratio?

No -- the S/F ratio (SpO2/FiO2) is an approximation of the P/F ratio (PaO2/FiO2). The P/F ratio uses the arterial oxygen partial pressure (PaO2) measured by arterial blood gas, which is the gold standard. The S/F ratio substitutes SpO2 (from pulse oximetry) as a non-invasive approximation. Rice et al validated that S/F thresholds of 315, 235 and 148 correspond to P/F thresholds of 300, 200 and 100. The S/F is useful prehospitally where ABG is not available. Illustrative example only.

When should I escalate from supplemental oxygen to CPAP or positive pressure ventilation?

Escalation decisions depend on clinical assessment alongside the S/F ratio: consider CPAP when a patient has an S/F below 315 despite high-flow oxygen, is conscious and cooperative, and has an appropriate clinical indication (pulmonary oedema, COPD exacerbation). Consider positive pressure ventilation (BVM or intubation) when the patient cannot protect their airway, has severe hypoxaemia (S/F <148) not responding to high-flow oxygen, has increasing work of breathing suggesting impending fatigue, or has a declining level of consciousness. Follow your agency's clinical practice guidelines and consider early medical direction contact for escalation decisions. Illustrative example only.

SpO₂ / FiO₂ Ratio (S/F) — Free Paramedics Calculator

🎯

A patient with known COPD is on high-flow oxygen at 15 L/min (FiO2 approximately 60%) and the SpO2 reads 92%. Before deciding whether to escalate to CPAP or BVM assist, the crew needs the S/F ratio to classify the severity of hypoxaemia against ARDS criteria.

SpO₂ / FiO₂ Ratio (S/F)

Respiratory

SpO₂ (%)

Pulse oximetry reading

FiO₂ (%)

21% = room air · 44% = 4 L/min NC

S/F Ratio = SpO₂ ÷ FiO₂ (as decimal) FiO₂ guide: Room air 21% · Nasal cannula 1L=24%, 2L=28%, 4L=36%, 6L=44% · Simple mask ~40% · NRB mask ~60–80% · BVM 100%
S/F thresholds (ARDS Berlin): ≥315 Normal · 235–314 Mild · 148–234 Moderate · <148 Severe

Reference: Rice TW et al. — Critical Care Medicine 2007 · ARDS Berlin Definition 2012

⚕️ Clinical safety: 🇦🇺 Verify with facility drug formulary and senior clinician · Meets AHPRA/ACSQHC standards

1 What this calculator does

Calculates the SpO2/FiO2 (S/F) ratio, an approximation of the PaO2/FiO2 (P/F) ratio used for ARDS classification. Classifies oxygenation status from normal to severe hypoxaemia and provides recommended prehospital interventions for each threshold.

2 Formula & professional reasoning

S/F ratio = SpO2 (%) / (FiO2 as a decimal)
FiO2 reference:
  Room air: 21% | Low-flow 2L: ~28% | Low-flow 4L: ~36% | High-flow 15L: ~60%
  Simple face mask: ~40-60% | Non-rebreather mask 15L: ~80-95% | Intubated/BVM 100%: 100%
S/F thresholds:
  >=315: Normal oxygenation
  235-314: Mild hypoxaemia (ARDS mild equivalent)
  148-234: Moderate hypoxaemia
  <148: Severe hypoxaemia

The S/F ratio is a non-invasive surrogate for the PaO2/FiO2 (P/F) ratio used in the Berlin ARDS definition, because SpO2 can be measured non-invasively in the prehospital setting while PaO2 requires arterial blood gas sampling. Rice et al (2007) established a correlation between S/F and P/F ratios, with S/F thresholds of 315, 235 and 148 corresponding to P/F thresholds of 300, 200 and 100 (mild, moderate and severe ARDS). This allows prehospital clinicians to estimate ARDS severity and escalate airway support appropriately before arterial blood gas results are available.

3 Worked examples

⚠️ Illustrative example only — not clinical or professional instruction.

Basic

COPD patient on high-flow oxygen

Given: SpO2: 92% | Oxygen delivery: high-flow non-rebreather mask 15 L/min | FiO2: approximately 80%

Working: FiO2 as decimal: 80/100 = 0.80 | S/F ratio: 92/0.80 = 115

Answer: S/F ratio: 115 -- Severe hypoxaemia (<148). High-flow O2 not adequate. CPAP or positive pressure ventilation indicated.

💡 Illustrative example only. Verify all clinical decisions with your agency protocol, medical director and partner clinician. Meets AHPRA/ACSQHC prehospital care standards. An SpO2 of 92% on an NRB mask (FiO2 ~80%) demonstrates severe hypoxaemia -- the patient is receiving near-maximal non-invasive oxygen and still has a very low S/F ratio. Escalation to CPAP or BVM is indicated. Notify receiving hospital of impending respiratory failure.

Standard

Post-resuscitation patient on NRB mask

Given: SpO2: 98% | FiO2: 85% (NRB mask 15 L/min) | Clinical context: ROSC after cardiac arrest

Working: S/F ratio: 98/0.85 = 115.3

Answer: S/F ratio: 115 -- Severe hypoxaemia category despite high SpO2, due to high FiO2 requirement

💡 Illustrative example only. Verify all clinical decisions with your agency protocol, medical director and partner clinician. Meets AHPRA/ACSQHC prehospital care standards. This scenario illustrates why SpO2 alone is misleading. A reading of 98% appears adequate but requires a very high FiO2 -- the S/F of 115 indicates severe impairment of gas exchange. This patient requires close monitoring and may need escalation to ventilatory support.

Advanced

Normal oxygenation on room air

Given: SpO2: 98% | FiO2: 21% (room air) | No supplemental oxygen

Working: FiO2 as decimal: 21/100 = 0.21 | S/F ratio: 98/0.21 = 466.7

Answer: S/F ratio: 467 -- Normal oxygenation. No supplemental oxygen required.

💡 Illustrative example only. Verify all clinical decisions with your agency protocol, medical director and partner clinician. Meets AHPRA/ACSQHC prehospital care standards. SpO2 of 98% on room air gives a high S/F ratio confirming normal gas exchange efficiency. This is the expected baseline for a healthy adult. Use as a comparison point -- if this patient later deteriorates to SpO2 95% requiring supplemental oxygen, the S/F will reveal the true severity of the gas exchange impairment.

4 Sanity check

FiO2 reference by delivery method (approximate)

Actual FiO2 delivery is patient-dependent -- minute ventilation, fit of mask and respiratory pattern all affect delivered FiO2.

S/F ratio thresholds (Berlin ARDS equivalents)

>=315: Normal (P/F equiv >300) | 235-314: Mild ARDS (P/F 200-300) | 148-234: Moderate ARDS (P/F 100-200) | <148: Severe ARDS (P/F <100)

S/F is an approximation -- gold standard remains PaO2/FiO2 on arterial blood gas.

SpO2 reliability

SpO2 is unreliable in: poor perfusion (shock, cold extremities), nail polish or acrylic nails, carbon monoxide poisoning (falsely normal), severe anaemia | Obtain from a well-perfused digit with good waveform

S/F is not validated as a standalone clinical decision tool

S/F ratio is an approximation and one data point among many -- it supports but does not replace clinical assessment of work of breathing, mental status and clinical trajectory

5 Common errors

Error	Cause	Consequence	Fix
Using SpO2 alone without calculating the FiO2 requirement to assess oxygenation	Treating SpO2 as the only oxygenation metric	Missing significant gas exchange impairment -- an SpO2 of 96% on 80% FiO2 is far more concerning than 96% on room air	Always consider SpO2 in the context of the FiO2 being delivered. An SpO2 that requires high FiO2 to maintain indicates impaired gas exchange that SpO2 alone does not reveal.
Using carbon monoxide poisoning patients' SpO2 for S/F calculation	Not recognising that pulse oximetry cannot distinguish oxyhaemoglobin from carboxyhaemoglobin	Falsely reassuring SpO2 leads to underestimation of hypoxia severity	In suspected CO poisoning, SpO2 is unreliable -- the oximeter reads COHb as oxyhaemoglobin, giving falsely normal readings. Use clinical signs and CO-oximetry if available. The S/F ratio is not valid in CO poisoning.
Applying ARDS criteria to a non-ARDS cause of hypoxaemia without context	Using S/F thresholds as definitive diagnoses rather than severity classifiers	Inappropriate escalation or de-escalation decisions based on a single ratio	The S/F ratio classifies severity of hypoxaemia -- it does not identify the cause. The treatment of hypoxaemia depends on the underlying cause (pneumonia, pulmonary oedema, bronchospasm, PE) as well as severity. Integrate the S/F with the clinical presentation.
Not accounting for the unreliability of SpO2 in poor peripheral perfusion	Accepting an SpO2 reading without checking the plethysmographic waveform quality	Inaccurate SpO2 used for S/F calculation -- may be much lower or higher than actual	Only use SpO2 readings with a good-quality plethysmographic waveform. In shock or poor perfusion, move the probe to a more central location (ear lobe, forehead sensor) or interpret SpO2 readings with significant caution.