SpO2/FiO2 (S/F) ratio: how to calculate it from pulse oximetry and FiO2
The SpO2/FiO2 (S/F) ratio is the pulse-oximetry oxygen saturation, as a percentage, divided by the inspired oxygen fraction. For an SpO2 of 95% at an FiO2 of 0.21, the S/F ratio is 95 ÷ 0.21 = 452.4.
Not a medical device. Not a diagnosis or dosing recommendation.
What is the SpO2/FiO2 ratio?
The S/F ratio relates how saturated the blood is, as read by a pulse oximeter, to how much oxygen the patient is breathing. It was studied as a stand-in for the PaO2/FiO2 (P/F) ratio, which needs an arterial blood gas.
Published work describes the P/F ratio as an invasive measurement that requires arterial blood gas sampling, and the S/F ratio as a non-invasive tool based on pulse oximetry that gives a cost-effective and rapid way to monitor oxygenation.[1]
It's also written as SF ratio, SpO2:FiO2, or "SpO2 over FiO2". This page explains the arithmetic. It doesn't give a normal range, and it doesn't apply any ARDS category.
The S/F ratio formula
sf_ratio = spo2_percent / fio2In words: divide the SpO2 percentage by the inspired oxygen fraction. Magent's calculator reports the result to 1 decimal place, as a plain number.
What each part means
| Term | Plain-English meaning | Unit | Accepted range |
|---|---|---|---|
spo2_percent | Oxygen saturation from a pulse oximeter | % | 50–100 |
fio2 | Fraction of inspired oxygen | fraction, not a percent | 0.21–1.0, required |
sf_ratio | SpO2 ÷ FiO2 | plain number | 1 decimal place |
FiO2 has to be given. The calculator doesn't assume a value for it.
How the formula works
- Take the SpO2 as a percentage, for example 95.
- Put the FiO2 as a fraction. If you have a percentage, divide by 100 to get the fraction.
- Divide the SpO2 by the FiO2.
- Round to 1 decimal place.
The ratio moves in opposite directions with its two inputs. A higher SpO2 raises it. A higher FiO2 lowers it. At an FiO2 of 1.0 the ratio equals the SpO2, because dividing by 1 changes nothing. At an FiO2 of 0.50 the ratio is twice the SpO2.
Units
SpO2 goes in as a percentage, not a fraction. FiO2 goes in as a fraction, not a percentage. Mixing those up changes the answer by a factor of 100.
Worked examples
Examples 1 to 3 use Magent's own expected values for the calculator; Example 4 is computed from the formula.
Example 1: SpO2 95%, FiO2 0.21
- 95 ÷ 0.21 = 452.380…
- Rounded to 1 decimal: 452.4
Example 2: SpO2 95%, FiO2 1.0
- 95 ÷ 1.0 = 95
- Rounded to 1 decimal: 95.0
Example 3: SpO2 50%, FiO2 0.21
- 50 ÷ 0.21 = 238.095…
- Rounded to 1 decimal: 238.1
Example 4: SpO2 90%, FiO2 0.40
- 90 ÷ 0.40 = 225
- Rounded to 1 decimal: 225.0
Examples 1 and 2 share the same SpO2. Raising FiO2 from 0.21 to 1.0 brings the ratio down from 452.4 to 95.0, so the FiO2 matters as much as the saturation.
S/F ratio reference table
These values are computed from the formula and rounded to 1 decimal, as Magent's calculator reports them. They show how the ratio moves; they aren't targets, reference ranges or categories.
| SpO2 \ FiO2 | 0.21 | 0.28 | 0.35 | 0.40 | 0.50 | 0.60 | 1.00 |
|---|---|---|---|---|---|---|---|
| 88% | 419.0 | 314.3 | 251.4 | 220.0 | 176.0 | 146.7 | 88.0 |
| 90% | 428.6 | 321.4 | 257.1 | 225.0 | 180.0 | 150.0 | 90.0 |
| 92% | 438.1 | 328.6 | 262.9 | 230.0 | 184.0 | 153.3 | 92.0 |
| 94% | 447.6 | 335.7 | 268.6 | 235.0 | 188.0 | 156.7 | 94.0 |
| 96% | 457.1 | 342.9 | 274.3 | 240.0 | 192.0 | 160.0 | 96.0 |
Find the SpO2 row and the FiO2 column. The 1.00 column equals the SpO2, and each value in the 0.50 column is twice the SpO2.
History of the S/F ratio
Magent's calculator cites T. W. Rice, A. P. Wheeler, G. R. Bernard, D. L. Hayden, D. A. Schoenfeld and L. B. Ware, Comparison of the SpO2/FIO2 ratio and the PaO2/FIO2 ratio in patients with acute lung injury or ARDS, published in Chest in 2007 (volume 132, issue 2, pages 410–417).
The abstract explains the starting point. The criteria for acute lung injury (ALI) and ARDS used the P/F ratio from an arterial blood gas, and the authors set out to test whether the S/F ratio could be substituted for it when assessing oxygenation. They compared paired SpO2 and PaO2 measurements, with SpO2 values of 97% or less, from 672 patients in an ARDS Network trial of a lower tidal volume ventilator strategy. A second ARDS Network trial of 402 patients supplied a confirmation data set. The authors concluded that S/F ratios correlate with P/F ratios.
The paper also reports S/F values that line up with specific P/F values. This page doesn't reproduce them.
When it's used
Published work describes the S/F ratio being used as a surrogate for the P/F ratio, and as a tool in acute lung injury and ARDS.[1] A scoping analysis of 32 articles found it studied in adults, children and newborns in critical care, with common conditions including acute hypoxemic respiratory failure and ARDS.[1]
Where it's used
The S/F ratio needs a pulse oximeter and a known FiO2, not a blood sample. Published work highlights its use in settings where more advanced methods are unavailable.[1] The studies it draws on come mainly from critical care units, with some from emergency departments.[1]
Who uses it
The sources cited here are the original Rice paper and a scoping analysis of later studies. They don't single out one professional group, so this page doesn't name one.
Limits of the calculation
- It's a ratio of two numbers. It isn't the P/F ratio, it doesn't apply ARDS or Berlin severity categories, and it isn't the ROX index.
- FiO2 is required as a fraction from 0.21 to 1.0. It isn't defaulted to room air.
- Pulse-oximetry readings can be affected by skin pigmentation, nail polish, shock, anemia and conditions such as methemoglobinemia, and can be misleading in carbon monoxide poisoning.[1]
- At saturations above 97%, readings sit on the flat part of the oxygen dissociation curve, and pulse oximetry tops out at 100%.[1] Rice's analysis used SpO2 values of 97% or less.
- Inputs outside the accepted ranges are rejected, not estimated.
Clinical decisions belong to a qualified clinician who knows the patient.
How it differs from related calculators
- PaO2/FiO2 ratio divides arterial PO2 in mm Hg by the inspired oxygen fraction. It needs an arterial blood gas; the S/F ratio uses pulse oximetry instead.
- ROX index takes the S/F ratio and divides it again by the respiratory rate.
Glossary
- SpO2: oxygen saturation measured by a pulse oximeter, as a percentage.
- FiO2: the fraction of inspired oxygen, from 0.21 to 1.0.
- PaO2: the partial pressure of oxygen in arterial blood, from a blood gas.
- P/F ratio: PaO2 divided by FiO2.
- ARDS: acute respiratory distress syndrome.
Common mistakes
- Entering FiO2 as a percentage. Use the fraction, not the percentage.
- Entering SpO2 as a fraction. Use the percentage.
- Treating S/F as P/F. They're different ratios built from different measurements.
- Leaving FiO2 out. The calculator needs it and won't assume one.
Source
Rice TW, Wheeler AP, Bernard GR, Hayden DL, Schoenfeld DA, Ware LB. Comparison of the SpO2/FIO2 ratio and the PaO2/FIO2 ratio in patients with acute lung injury or ARDS. Chest. 2007;132(2):410-417. DOI: 10.1378/chest.07-0617
Further reading
- Reddy M, Kulkarni M, Kanakalakshmi ST, Shenoy L, KrishnaBhat RR. Non-invasive SpO2/FiO2 ratio (SFR) as surrogate for PaO2/FiO2 ratio (PFR): A scoping review. J Crit Care Med (Targu Mures). 2025;11(3):221-232.
Frequently asked questions
How do you calculate the S/F ratio?
Divide the SpO2 percentage by the FiO2 as a fraction. For 95% at 0.21, that's 95 ÷ 0.21 = 452.4.
What is the SpO2/FiO2 ratio?
It's pulse-oximetry oxygen saturation divided by inspired oxygen fraction, studied as a non-invasive stand-in for the P/F ratio.[1]
Is the S/F ratio the same as the P/F ratio?
No. The P/F ratio uses arterial PO2 from a blood gas. The S/F ratio uses SpO2 from a pulse oximeter. Rice and colleagues reported that the two correlate.
Should FiO2 be a percentage or a fraction?
A fraction. Divide a percentage by 100 first.
What is a normal S/F ratio?
This page doesn't give a normal range, an ARDS category or a cutoff. It calculates the ratio; what a value means depends on the clinical context.
What is the S/F ratio at an FiO2 of 1.0?
It equals the SpO2. For SpO2 95% at FiO2 1.0, the ratio is 95.0.