What this calculator does
PaO₂/FiO₂ ratio (Horowitz index) works out oxygenation index used to gauge the severity of acute respiratory failure. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
FormulaPaO2/FiO2 ratio = arterial PaO2(mmHg) / fraction of inspired oxygen
The inputs explained
| Field | What to enter |
|---|---|
| Arterial PaO₂ (mmHg) | A number, measured in mmHg. Starts at 90. |
| FiO₂ (%) | A number, measured in %. Starts at 40. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with arterial pao₂
Every other input is held at the calculator’s starting values while arterial pao₂ varies. Select any row to load that scenario into the calculator.
How the answer changes with arterial pao₂
| Arterial PaO₂ (mmHg) | PaO₂/FiO₂ ratio | Reading | Note |
|---|---|---|---|
| 45 | 113 | 100 - 200: moderate impairment (moderate ARDS range) | Part of the Berlin definition of ARDS severity: interpretation needs the full clinical picture, not this number alone. |
| 67.5 | 169 | 100 - 200: moderate impairment (moderate ARDS range) | Part of the Berlin definition of ARDS severity: interpretation needs the full clinical picture, not this number alone. |
| 90 | 225 | 200 - 300: mild impairment (mild ARDS range) | Part of the Berlin definition of ARDS severity: interpretation needs the full clinical picture, not this number alone. |
| 135 | 338 | Above 300: no significant oxygenation impairment by the Berlin ARDS criteria | Part of the Berlin definition of ARDS severity: interpretation needs the full clinical picture, not this number alone. |
| 180 | 450 | Above 300: no significant oxygenation impairment by the Berlin ARDS criteria | Part of the Berlin definition of ARDS severity: interpretation needs the full clinical picture, not this number alone. |
| 270 | 675 | Above 300: no significant oxygenation impairment by the Berlin ARDS criteria | Part of the Berlin definition of ARDS severity: interpretation needs the full clinical picture, not this number alone. |