PaO2/FiO2 Ratio Calculator

Severity	P/F Ratio
No ARDS by oxygenation criterion	> 300
Mild ARDS	200 – 300
Moderate ARDS	100 – 200
Severe ARDS	≤ 100

Criterion	Requirement
Timing	Onset within 1 week of a known clinical insult or new/worsening respiratory symptoms
Chest imaging	Bilateral opacities on X-ray or CT not fully explained by effusions, lobar/lung collapse, or nodules
Origin of edema	Respiratory failure not fully explained by cardiac failure or fluid overload
Oxygenation	P/F ratio ≤ 300 with PEEP or CPAP ≥ 5 cmH2O (mild 200–300, moderate 100–200, severe ≤ 100)

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Walk-through

How to Use This Calculator

3 steps

1

Enter PaO2

Enter the arterial oxygen partial pressure (PaO2) from a blood gas result, in mmHg. This is the patient's measured PaO2 at the moment of interest, not a normal-range reference value.

2

Enter FiO2

Enter the fraction of inspired oxygen (FiO2) the patient was breathing when the blood gas was drawn, as a decimal from 0.21 (room air) to 1.0 (100% oxygen). The field defaults to 0.21 for room air — replace it with the delivered FiO2 whenever supplemental oxygen or a ventilator is in use.

3

Read the P/F ratio and severity

The calculator divides PaO2 by FiO2 instantly and classifies the result into a Berlin ARDS severity tier — mild (200-300), moderate (100-200), severe (≤100), or above 300 (outside the ARDS oxygenation range). Check the ARDS Severity tab to see your ratio highlighted against every tier, and the Berlin Criteria Note tab for the full four-part diagnostic definition.

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Reference

Formula & Methodology

1 formula

P/F ratio

P/F = PaO2 ÷ FiO2

PaO2 is the arterial oxygen partial pressure in mmHg, measured directly from an arterial blood gas. FiO2 is the fraction of inspired oxygen the patient is breathing, expressed as a decimal between 0.21 (room air) and 1.0 (100% oxygen). The result is a unitless ratio used by the Berlin Definition of ARDS to stage oxygenation impairment.

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High YMYL

Trust, Methodology & Sources

Reviewed by Calculover Clinical Team · Updated 2026-07-22 · 1 sources

Editorial accountability

Author: Calculover Clinical Team - Clinical content team

Owner: Calculover Editorial - Editorial oversight

Reviewer: Calculover Clinical Team - Clinical reviewer

Last reviewed: 2026-07-22

Last verified: 2026-07-22

Data effective: 2026-07-22

Methodology

Implements the Berlin Definition of ARDS oxygenation criterion (ARDS Definition Task Force, Ranieri VM et al., JAMA 2012;307(23):2526-2533): P/F ratio = PaO2 ÷ FiO2, staged as mild (200-300), moderate (100-200), or severe (≤100).

Assumption: FiO2 is entered as a decimal fraction (0.21-1.0), not a percentage.

Assumption: The calculator assumes the entered PaO2 and FiO2 were measured concurrently, as required by the Berlin Definition.

Assumption: A minimum PEEP or CPAP of 5 cmH2O is assumed but not verified — the calculator does not collect ventilator settings.

Limitations & guidance

The P/F ratio is only the oxygenation criterion of the Berlin Definition; a full ARDS diagnosis also requires timing, chest-imaging, and cardiac-exclusion criteria that this calculator does not assess.

This tool does not verify that PEEP/CPAP was at least 5 cmH2O at the time of measurement, which the Berlin Definition requires for a valid severity classification.

Results are for clinical decision support and education only and do not replace a treating clinician's full assessment.

Professional guidance: This calculator is for clinical decision support and educational purposes only and is not a substitute for professional medical judgment, diagnosis, or treatment. Always consult a qualified clinician for patient care decisions.

Primary sources

ARDS Definition Task Force. Acute Respiratory Distress Syndrome: The Berlin Definition. JAMA. 2012;307(23):2526-2533.

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Glossary

Key Terms Explained

6 terms

P/F ratio The PaO2/FiO2 ratio — arterial oxygen partial pressure divided by the fraction of inspired oxygen. It is the oxygenation criterion used to stage ARDS severity under the Berlin Definition.

Berlin Definition The 2012 consensus definition of ARDS (ARDS Definition Task Force, JAMA 2012), which replaced the 1994 American-European Consensus Conference definition. It defines ARDS using four criteria: timing, chest imaging, origin of edema, and oxygenation (P/F ratio with a minimum PEEP).

ARDS Acute respiratory distress syndrome — a form of acute, diffuse lung injury causing severe hypoxemia, characterized by bilateral pulmonary infiltrates and reduced lung compliance, not fully explained by cardiac failure or fluid overload.

PEEP Positive end-expiratory pressure — the pressure maintained in the airways at the end of exhalation, either from a ventilator or CPAP device. The Berlin Definition requires a minimum PEEP (or CPAP) of 5 cmH2O when the P/F ratio is measured, so severity staging reflects true gas-exchange impairment rather than under-recruited lungs.

Hypoxemia Abnormally low arterial oxygen levels. A falling P/F ratio is one way to quantify the severity of hypoxemia in a patient with suspected ARDS.

FiO2 Fraction of inspired oxygen — the concentration of oxygen a patient is breathing, expressed as a decimal from 0.21 (room air, roughly 21% oxygen) to 1.0 (100% oxygen delivered by a ventilator or high-flow device).

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Scenarios

Real-World Examples

3 worked examples

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Mild ARDS

PaO2 175 mmHg on FiO2 0.7

PaO2 175 mmHg FiO2 0.7

175 ÷ 0.7 = 250. A P/F ratio of 250 falls in the mild ARDS range (200-300). This is the least severe Berlin tier, but still meets the oxygenation criterion for ARDS if the timing, imaging, and cardiac-exclusion criteria are also met.

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Moderate ARDS

PaO2 90 mmHg on FiO2 0.6

PaO2 90 mmHg FiO2 0.6

90 ÷ 0.6 = 150. A P/F ratio of 150 falls in the moderate ARDS range (100-200), reflecting more significant gas-exchange impairment than the mild tier.

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Severe ARDS

PaO2 72 mmHg on FiO2 0.9

PaO2 72 mmHg FiO2 0.9

72 ÷ 0.9 = 80. A P/F ratio of 80 falls in the severe ARDS range (≤100), the highest-severity Berlin tier, indicating profound hypoxemia relative to the delivered oxygen concentration.

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Deep Dive

Understanding the PaO2/FiO2 Ratio

3 chapters · 4 min read

The PaO2/FiO2 (P/F) ratio is the bedside oxygenation measurement clinicians use to stage the severity of acute respiratory distress syndrome (ARDS) under the Berlin Definition. This calculator computes the ratio from a patient's arterial blood gas and classifies it into the mild, moderate, or severe tier.

How the P/F ratio works

The P/F ratio is simply PaO2 divided by FiO2. PaO2 comes directly from an arterial blood gas; FiO2 is the fraction of oxygen the patient is breathing, from 0.21 on room air up to 1.0 on 100% supplemental oxygen. Because the ratio normalizes PaO2 against how much oxygen the patient is receiving, it gives a more comparable measure of lung function than PaO2 alone — a PaO2 of 90 mmHg means something very different on room air versus on 90% FiO2.

The Berlin Definition (ARDS Definition Task Force, JAMA 2012) uses the ratio to stage ARDS severity: mild is 200 to 300, moderate is 100 to 200, and severe is 100 or below. Lower ratios indicate more severe impairment of gas exchange.

Inputs and what they mean

PaO2 (mmHg) is the arterial oxygen partial pressure measured from a blood gas. Normal room-air values are roughly 80-100 mmHg; values are read directly off the lab result, not calculated.

FiO2 (fraction) is the concentration of oxygen the patient is breathing, as a decimal. Room air is 0.21; a nasal cannula, mask, or ventilator delivers a higher FiO2 that should be documented at the time the blood gas is drawn. Using the wrong FiO2 — for example, forgetting the patient is on supplemental oxygen — will produce a misleadingly high or low ratio.

Limits and edge cases

The P/F ratio is only one of four Berlin Definition criteria. A P/F ratio at or below 300 does not by itself diagnose ARDS — the patient must also have symptom onset within one week of a known clinical insult, bilateral opacities on chest imaging not fully explained by effusions or collapse, and respiratory failure not fully explained by cardiac failure or fluid overload. All three severity tiers additionally require a minimum PEEP (or CPAP) of 5 cmH2O at the time PaO2 is measured — a ratio measured without adequate PEEP can understate true lung function. This calculator does not verify PEEP, imaging, timing, or cardiac status; it computes the oxygenation ratio only, and the final ARDS diagnosis always requires a clinician's full assessment.

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Questions

Frequently Asked Questions

6 questions

What is the PaO2/FiO2 ratio formula?+

The P/F ratio is arterial oxygen partial pressure (PaO2, in mmHg) divided by the fraction of inspired oxygen (FiO2, expressed as a decimal from 0.21 to 1.0). For example, a PaO2 of 150 mmHg on an FiO2 of 0.5 gives a P/F ratio of 300.

What are the Berlin ARDS severity cutoffs?+

The Berlin Definition stages ARDS by P/F ratio: mild is 200 to 300, moderate is 100 to 200, and severe is 100 or below. A P/F ratio above 300 falls outside the ARDS oxygenation criterion entirely.

Why does the Berlin Definition require a minimum PEEP?+

Positive end-expiratory pressure (PEEP) recruits collapsed alveoli and improves oxygenation, so a P/F ratio measured without adequate PEEP can understate true lung function. The Berlin Definition requires the ratio be measured at a PEEP or CPAP of at least 5 cmH2O so severity staging reflects genuine gas-exchange impairment rather than under-recruited lungs.

What FiO2 should I use for a patient on room air?+

Room air is approximately 21% oxygen, so use an FiO2 of 0.21. This calculator defaults the FiO2 field to 0.21 for that reason — replace it with the patient's actual delivered FiO2 whenever supplemental oxygen or mechanical ventilation is in use.

How is the P/F ratio different from the A-a gradient?+

The P/F ratio is a simple division that only needs PaO2 and FiO2, making it fast to calculate at the bedside and the basis of the Berlin ARDS severity tiers. The alveolar-arterial (A-a) gradient additionally accounts for barometric pressure, PaCO2, and the respiratory quotient, giving a more complete picture of the alveolar-to-arterial oxygen difference but requiring more inputs.

Can the P/F ratio alone diagnose ARDS?+

No. The Berlin Definition requires four criteria together: onset within one week of a known clinical insult, bilateral opacities on chest imaging not fully explained by cardiac failure or fluid overload, respiratory failure not attributable to a cardiac cause, and a P/F ratio at or below 300 measured with PEEP/CPAP of at least 5 cmH2O. This calculator computes only the oxygenation ratio — a clinician must confirm the other three criteria.

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Blood Gas Values

P/F Ratio & Severity

Berlin ARDS Severity Tiers

The Full Berlin Definition of ARDS

How to Use This Calculator

Enter PaO2

Enter FiO2

Read the P/F ratio and severity

Formula & Methodology

Trust, Methodology & Sources

Key Terms Explained

Real-World Examples

Mild ARDS

Moderate ARDS

Severe ARDS

Understanding the PaO2/FiO2 Ratio

How the P/F ratio works

Inputs and what they mean

Limits and edge cases

Frequently Asked Questions

Get a branded PDF of your results

PaO2/FiO2 Ratio Calculator

Blood Gas Values

P/F Ratio & Severity

Berlin ARDS Severity Tiers

The Full Berlin Definition of ARDS

How to Use This Calculator

Enter PaO2

Enter FiO2

Read the P/F ratio and severity

Formula & Methodology

Trust, Methodology & Sources

Key Terms Explained

Real-World Examples

Understanding the PaO2/FiO2 Ratio

How the P/F ratio works

Inputs and what they mean

Limits and edge cases

Frequently Asked Questions

Get a branded PDF of your results

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