What is the S/F ratio used for?

The S/F (SpO2/FiO2) ratio is a non-invasive surrogate for the P/F (PaO2/FiO2) ratio. It helps classify ARDS severity (mild, moderate, severe) without requiring an arterial blood gas sample.

What are the ARDS cutoffs for the S/F ratio?

Based on Rice et al. (2007): S/F > 315 = no ARDS / mild hypoxemia; S/F 235–315 = mild ARDS (equivalent to P/F 200–300); S/F < 235 = moderate-to-severe ARDS (equivalent to P/F < 200).

Why is FiO2 entered as a percentage?

Room air is 21% oxygen (FiO2 = 0.21 = 21%). This calculator accepts FiO2 in percentage form (e.g., 40 for 40%) for clinical convenience, but it also accepts decimal values like 0.40 — the calculator auto-detects the format.

Is the S/F ratio accurate at very high or very low SpO2?

The S/F ratio is most accurate when SpO2 is between 80% and 97%. At SpO2 above 97%, the oxyhemoglobin dissociation curve is relatively flat, reducing the sensitivity of S/F as a P/F surrogate. Values above 97% may underestimate true hypoxemia.

Does this replace arterial blood gas measurement?

No. The S/F ratio is a screening tool and surrogate. Arterial blood gas (ABG) measurement remains the gold standard for definitive ARDS classification and ventilator management decisions.

SpO2/FiO2 Ratio Calculator

Name: SpO2/FiO2 Ratio Calculator
Availability: InStock
Author: Nham Vu

Enter SpO2 (%) and FiO2 (%) to calculate the S/F ratio and estimate ARDS severity per Berlin criteria.

Patient Parameters

SpO2 (pulse oximetry, %)

Valid range: 70 – 100%

FiO2 (fraction of inspired oxygen)

Enter as % (e.g. 40 for 40%) or decimal (e.g. 0.40)

Berlin Criteria — S/F Cutoffs

No ARDS

S/F > 315

Mild ARDS

235 < S/F ≤ 315

Moderate ARDS

118 < S/F ≤ 235

Severe ARDS

S/F ≤ 118

Reference: Rice et al. (2007), Crit Care Med. S/F cutoffs validated against Berlin P/F criteria.

Clinical note: This tool is for educational and screening purposes only. S/F ratio accuracy is reduced when SpO2 > 97%. Always confirm with ABG for definitive ARDS classification and ventilator management.

Summary

Enter SpO2 (%) and FiO2 (%) to calculate the S/F ratio and estimate ARDS severity per Berlin criteria.

How it works

Enter the patient's SpO2 reading from pulse oximetry (must be 70–100%).
Enter the FiO2 being delivered — as a percentage (21–100%) or a decimal (0.21–1.00).
The calculator divides SpO2 by FiO2 to produce the S/F ratio.
The result is compared against validated Berlin criteria cutoffs for ARDS classification.
An estimated equivalent P/F ratio is shown using the Rice et al. regression formula.

Use cases

Rapid bedside ARDS screening without arterial blood gas sampling.
Serial monitoring of oxygenation trends in ICU patients.
Triaging hypoxemia severity in resource-limited settings.
Estimating P/F ratio when arterial blood gas is not immediately available.
Teaching respiratory physiology and oxygenation indices.
Documenting ventilator management parameters in clinical notes.

Frequently Asked Questions

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