SpO2/FiO2 Ratio Calculator
Calculate the SpO2/FiO2 ratio as a non-invasive surrogate for PaO2/FiO2, useful for grading oxygenation when arterial blood gas results are unavailable.
🫁 Clinical Reference📐 S/F = SpO2 / FiO2; P/F ≈ (S/F − 64) / 0.84🏥 Health
SpO2 (%)
FiO2 (as decimal, e.g. 0.4)
Oxygen delivery (to estimate FiO2 if unknown)
Please enter valid values.
Formula & Reference
| Variable | Symbol | Formula | Units |
|---|---|---|---|
| SpO2/FiO2 Ratio Calculator | — | S/F = SpO2 / FiO2; P/F ≈ (S/F − 64) / 0.84 | ratio |
Step-by-Step Examples
Example 1
Room Air, Normal
SpO2 96% on room air (FiO2 0.21).
- S/F = 96 / 0.21
- S/F = 457
- Estimated P/F = (457 − 64) / 0.84 = 468
- Well above any ARDS threshold
✓ S/F 457 — normal oxygenation
Example 2
Moderate Impairment
SpO2 92% on FiO2 0.50.
- S/F = 92 / 0.50 = 184
- Estimated P/F = (184 − 64) / 0.84 = 143
- Falls in the moderate range
✓ S/F 184, estimated P/F 143
Example 3
Severe Impairment
SpO2 88% on FiO2 0.80.
- S/F = 88 / 0.80 = 110
- Estimated P/F = (110 − 64) / 0.84 = 55
- Corresponds to the severe range
✓ S/F 110, estimated P/F 55
Real-World Applications
ARDS Grading
The Berlin definition uses PaO2/FiO2, but S/F allows approximate severity grading without arterial sampling.
Resource-Limited Settings
Where arterial blood gas analysis is unavailable, S/F provides a workable substitute for oxygenation assessment.
Paediatric Care
S/F is used in paediatric critical care partly to reduce the need for arterial punctures in children.
Continuous Monitoring
Pulse oximetry is continuous, so S/F can track oxygenation trends between intermittent blood gas samples.
Common Mistakes to Avoid
⚠️
Using S/F when SpO2 exceeds 97%
The oxyhaemoglobin dissociation curve flattens above roughly 97%, so large PaO2 changes produce almost no SpO2 change. The ratio loses discrimination entirely in that range.
⚠️
Assuming a precise FiO2 from nasal cannula
FiO2 delivered by low-flow devices varies with the patient's respiratory rate and tidal volume. The conventional estimates are approximations only.
⚠️
Ignoring conditions that distort pulse oximetry
Carboxyhaemoglobin, methaemoglobin, poor perfusion, and skin pigmentation can all affect pulse oximetry accuracy, and therefore the ratio.
Frequently Asked Questions
What is the SpO2/FiO2 ratio? ▾
Peripheral oxygen saturation divided by the fraction of inspired oxygen — a non-invasive surrogate for the PaO2/FiO2 ratio used in ARDS grading.
How does S/F convert to P/F? ▾
A commonly used conversion is P/F equals S/F minus 64, divided by 0.84, derived from studies correlating the two measures.
What S/F thresholds correspond to ARDS severity? ▾
Roughly 315 corresponds to the mild ARDS threshold, 235 to moderate, and 145 to severe, mapping onto P/F values of 300, 200, and 100.
When is S/F unreliable? ▾
Above about 97% SpO2, where the dissociation curve flattens, and whenever pulse oximetry itself is compromised by poor perfusion or abnormal haemoglobin.
Can S/F replace arterial blood gas? ▾
Not entirely. It is useful for screening and trending, but formal ARDS diagnosis and detailed acid-base assessment still require arterial sampling.