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Oxygen Saturation Index Calculator

A non-invasive oxygenation index for grading respiratory failure severity.

How it is calculated

The tool computes OSI = FiO2 × MAP × 100 ÷ SpO2, with FiO2 as a fraction (so 60% is used as 0.60), MAP as the mean airway pressure in cmH2O and SpO2 in percent. You type FiO2 in percent. It also prints the SpO2/FiO2 ratio, SpO2 ÷ FiO2, rounded to a whole number. Grading: below 4 is shown as below the ARDS threshold, 4 to under 7.5 mild, 7.5 to under 12.3 moderate, and 12.3 or more severe. FiO2 and SpO2 must be above 0 and not more than 100, and MAP must be above 0. The OSI result is only meaningful for patients who are on a ventilator with a measured mean airway pressure.

Worked example

A ventilated child is on FiO2 60%, mean airway pressure 12 cmH2O, with SpO2 94%. OSI = 0.60 × 12 × 100 ÷ 94 = 720 ÷ 94 = 7.66. That is above 7.5 and below 12.3, so the grade is "Moderate". The S/F ratio is 94 ÷ 0.60 = 156.7, shown as "157 (SpO₂/FiO₂)". If the oxygen saturation fell to 88% with the same settings, OSI would rise to 8.18, still moderate.

Try this example →

Common mistakes

  • Typing FiO2 as a fraction (0.6) instead of percent (60): the tool then reads 0.6% and returns an OSI of 0.08, "Below ARDS threshold".
  • Using the arterial PaO2 instead of the pulse-oximeter SpO2; this index is built on the saturation reading.
  • Entering the peak or plateau pressure instead of the mean airway pressure, which overstates the index.
  • Reading "Below ARDS threshold" as proof of no lung injury; it only means this index is under 4.
  • Entering a mean airway pressure in other units (kPa, mmHg) or an implausible value like 150; the engine accepts any positive number and does not warn.

About this calculator

The Oxygen Saturation Index (OSI) is a non-invasive substitute for the traditional Oxygenation Index, calculated as (FiO₂ × mean airway pressure × 100) divided by SpO₂. It was developed because the classic Oxygenation Index needs an arterial blood gas to get PaO₂, which requires an arterial line — something not every ventilated patient, especially children, has in place.

Because pulse oximetry (SpO₂) is continuous and non-invasive, OSI lets clinicians in pediatric and neonatal intensive care grade the severity of respiratory failure and monitor it in real time without repeated blood draws. It is incorporated into the Pediatric Acute Lung Injury Consensus Conference (PALICC) criteria for grading ARDS severity in children, with rising OSI thresholds marking mild, moderate and severe disease.

This calculator takes FiO₂, mean airway pressure and SpO₂ and returns the OSI along with its corresponding severity grade.

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