Pao2 Fio2 Ratio Calculator - PF Ratio and ARDS Bands

The PaO2 FiO2 ratio (also called the P/F ratio or the Horowitz index) is the bedside number clinicians pull from an arterial blood gas to grade how well the lungs are oxygenating the blood relative to the oxygen being delivered. It is the oxygenation criterion in the Berlin Definition of acute respiratory distress syndrome (ARDS). A higher ratio means better oxygenation for the support being given; a lower ratio means the lung is falling behind.

• • ARDS severity grading at the bedside: Sorting an ABG into the mild (200 < P/F <= 300), moderate (100 < P/F <= 200), or severe (P/F <= 100) ARDS band on PEEP >= 5 cmH2O.
• • Trending oxygenation in the ICU: Tracking P/F over time on a fixed or changing FiO2 to see whether the lung is recruiting or drifting toward ECMO criteria.
• • Pre-oxygenation and intubation checks: Confirming a patient is not silently crossing into severe ARDS on a rising FiO2 before intubation or transport.
• • Education and protocol handoff: Documenting the exact ratio and band in chart notes and handoffs.

The PaO2 FiO2 ratio is not a measurement; it is a structured readout. The inputs come from an ABG and the device the patient is breathing from. The output maps to a severity band.

Use the calculator alongside the rest of the ABG. The ratio is most useful when read with the pH, PaCO2, A-a gradient, and the trend on the ventilator.

When the P/F ratio is no longer telling the whole story and the ventilator support is climbing, Oxygenation Index Calculator adds the mean airway pressure so the same ABG inputs return a bedside OI alongside the P/F ratio.

The calculator reads the arterial PaO2 in mmHg, the FiO2 as a decimal fraction (or a percent that is converted to a fraction), and the optional PEEP and atmospheric pressure, then runs the formulas that produce the P/F ratio, the Horowitz index, the Berlin ARDS severity band, and a short note.

• PaO2 (mmHg): Arterial partial pressure of oxygen from the ABG. Enter 20 to 600 mmHg.
• FiO2 (fraction): Fraction of inspired oxygen. 0.21 is room air; 0.4 to 1.0 is typical of supplemental oxygen and mechanical ventilation. Percent mode divides the entered value by 100 before the math.
• PEEP (cmH2O): Positive end-expiratory pressure on the ventilator. Used as a reminder that Berlin requires PEEP or CPAP >= 5 cmH2O; it does not change the ratio.
• Atmospheric pressure (mmHg): Ambient barometric pressure. Default 760 is sea level; lower values give an altitude-adjusted context note.

The atmospheric pressure input lets the same calculator serve altitude and aviation-medicine use cases. At 523 mmHg (about 3,000 m) a PaO2 of 60 mmHg on FiO2 0.4 still gives a P/F of 150, and the result panel shows an altitude-adjusted note.

According to Ranieri VM et al. - Acute Respiratory Distress Syndrome: The Berlin Definition (JAMA, 2012), the Berlin Definition grades ARDS severity by the PaO2/FiO2 ratio on PEEP or CPAP >= 5 cmH2O: mild 200 < P/F <= 300, moderate 100 < P/F <= 200, severe P/F <= 100.

When the same ABG needs to be read for both the lung-to-blood gradient and the oxygenation ratio, Aa Gradient Calculator pairs the alveolar gas equation with the P/F ratio on the same page.

Four ideas carry most of the clinical meaning of the result, and they are the same four to keep in mind.

These four ideas map directly onto the result panel and the worked examples. The band label tells you which Berlin category the ratio falls into, and the notes tell you whether the result is being read under the published PEEP condition.

When the ABG also needs to be read for acid-base status, Arterial Blood pH Calculator returns the pH, the bicarbonate, and the base excess from the same sample.

Treat the calculator as a structured readout of an arterial blood gas. The inputs come from the ABG and the bedside setup, and the outputs map to a clinical interpretation.

1. 1 Enter the PaO2 from the ABG: Use the arterial PaO2 in mmHg. Venous and capillary samples do not satisfy Berlin.
2. 2 Set the FiO2 and input mode: 0.21 for room air, 0.28 to 0.40 for low-flow cannula or simple mask, 0.4 to 0.6 for venturi or non-rebreather, up to 1.0 for mechanical ventilation.
3. 3 Enter PEEP and atmospheric pressure: PEEP from the ventilator, or 5 cmH2O for a patient on CPAP. Lower pressure (400 to 700 mmHg) is for altitude cases.
4. 4 Read the P/F ratio and the ARDS band: The result panel shows the ratio, band, Horowitz index (same number, second label), effective FiO2, and a short note.
5. 5 Cross-check with the rest of the ABG: Pair the P/F with the PaCO2, pH, and A-a gradient so oxygenation, ventilation, and acid-base are in the chart together.

A 60-year-old on a 50% mask has PaO2 95 mmHg, FiO2 0.5, PEEP 5 cmH2O, sea level. P/F = 95 / 0.5 = 190, in the moderate ARDS band.

When the same ABG also needs the Henderson-Hasselbalch read on bicarbonate, base excess, and anion gap, Acid Base Calculator returns the acid-base picture from the same sample alongside the P/F ratio.

A bedside calculator turns a five-line ABG into a structured answer that is easy to chart.

• • Fast ARDS severity grading: Returns the Berlin severity band in a single step, so the chart note and handoff use the same published language.
• • FiO2 input flexibility: Accepts FiO2 as a decimal fraction (0.21 to 1.0) or as a percent (21 to 100), matching device labels and ventilator screens.
• • Horowitz index alongside the ratio: Shows the same PaO2/FiO2 number under its clinical name, so a chart note can use either label.
• • Altitude and PEEP context: Adds an altitude-adjusted note for barometric pressures below 600 mmHg and a PEEP reminder below 5 cmH2O.
• • Reproducible documentation: Shows the inputs, ratio, band, and Horowitz index together, so a later reviewer can reproduce the work.

Most bedside cards give the formula and bands in a single line. A calculator that holds the full input set and band label is more useful for ICU, ED, and rapid-response use.

When the same admission also needs a multi-system severity score, Apache II Calculator returns the APACHE II total from the same ICU data set.

Several variables change the PaO2 FiO2 ratio, and the calculator surfaces the most important ones in the result panel.

• • The ratio is a bedside estimate, not a measurement. It assumes the FiO2 on the label is the FiO2 the patient is actually breathing; a poorly fitting mask can give a misleadingly high PaO2.
• • Berlin bands were derived in adults on PEEP or CPAP >= 5 cmH2O. Pediatric and neonatal practice uses the oxygen saturation index (OSI) for the same question.
• • A single value does not diagnose ARDS. Imaging, clinical context, and exclusion of alternative causes are part of the full Berlin criteria.

The calculator is meant to be read with the clinical picture, imaging, and trend over time. A single result does not diagnose ARDS, pulmonary embolism, or heart failure; it points the next step of the workup in a useful direction. Re-check the inputs against the ABG report and the device label before any treatment.

According to MDCalc - P/F Ratio (PaO2/FiO2), the P/F ratio equals PaO2 in mmHg divided by FiO2 as a fraction, and the Berlin ARDS severity band is read directly from the resulting number.

When the same ICU chart needs weight-based dosing for severe-ARDS supportive therapy, Body Surface Area Calculator returns the BSA from the same height and weight used for the ventilator settings.