P/F Ratio Calculator (PaO₂/FiO₂)
Calculate arterial PaO₂/FiO₂ ratio with explicit PaO₂ and FiO₂ units, FiO₂ reliability guidance, and ARDS definition limits.
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About
This page calculates the P/F ratio for one paired oxygenation condition: arterial oxygen tension (PaO₂) divided by inspired oxygen fraction (FiO₂). PaO₂ must come from an arterial sample, and FiO₂ must describe the same or closely matched time point. The result is an oxygenation ratio—a measure of arterial oxygenation relative to FiO₂—not a diagnosis of acute respiratory distress syndrome (ARDS), a direct measure of tissue oxygen delivery, or a treatment instruction. [3, 5, 8]
Interpretation
What the ratio describes
PaO₂ is the oxygen partial pressure measured in arterial blood. FiO₂ is the fraction of oxygen in inspired gas. Dividing PaO₂ by FiO₂ provides a compact index of arterial oxygenation under the stated oxygen condition; it does not identify the cause of impaired oxygenation. [3, 8]
Use paired measurements
Use PaO₂ and FiO₂ from the same or closely matched time point, and document oxygen device, flow, PEEP or CPAP, ventilation mode, and position when relevant. Do not substitute SpO₂, venous PO₂, capillary PO₂, or an estimated PaO₂ for the arterial value. [4, 8]
FiO₂ may be uncertain
Use a device-set or otherwise reliably known FiO₂ when available. Actual FiO₂ from a variable-performance oxygen device can differ with fit, leaks, inspiratory flow or demand, breathing pattern, and other factors. If FiO₂ is estimated, interpret P/F with that uncertainty. This calculator cannot verify the delivered FiO₂ and never infers FiO₂ from oxygen flow alone. [3, 4]
Berlin definition context
The 2012 Berlin definition uses mild 200 < P/F ≤ 300 (about 40 kPa), moderate 100 < P/F ≤ 200 (about 26.7 kPa), and severe P/F ≤ 100 (about 13.3 kPa). Mild Berlin ARDS permits PEEP or CPAP ≥5 cm H₂O. Moderate and severe Berlin ARDS require PEEP ≥5 cm H₂O. These are static definition context only. Acute timing, bilateral imaging findings, edema not fully explained by cardiac failure or fluid overload, and respiratory-support requirements also matter. This calculator does not assign a Berlin severity category. [1, 5]
- Static Berlin context
- Mild
- P/F ratio
- 200 < P/F ≤ 300 (about 40 kPa)
- Respiratory-support context
- PEEP or CPAP ≥5 cm H₂O
- Static Berlin context
- Moderate
- P/F ratio
- 100 < P/F ≤ 200 (about 26.7 kPa)
- Respiratory-support context
- PEEP ≥5 cm H₂O
- Static Berlin context
- Severe
- P/F ratio
- P/F ≤100 (about 13.3 kPa)
- Respiratory-support context
- PEEP ≥5 cm H₂O
2024 New Global Definition context
The New Global Definition uses P/F ≤300 mmHg and permits S/F ≤315 only when SpO₂ is ≤97%. Its non-intubated category can use P/F ≤300 with HFNO ≥30 L/min or NIV/CPAP with at least 5 cm H₂O expiratory pressure; it is not automatically divided into mild, moderate, or severe bands by this result. For intubated ARDS, its numerical P/F bands retain mild 200 < P/F ≤300, moderate 100 < P/F ≤200, and severe P/F ≤100. For New Global Definition context, obtain the paired measurement while the patient is comfortably at rest and at least 30 minutes after a change in position, FiO₂, or oxygen flow. Complete ARDS definition assessment also requires an acute predisposing risk factor, acute onset or worsening within one week, qualifying bilateral imaging abnormalities, pulmonary edema not primarily explained by cardiogenic edema or fluid overload, and the applicable respiratory-support setting. This is definition measurement context, not a universal wait or treatment instruction; urgent care should not be delayed. This page does not calculate S/F or evaluate those other definition requirements. [2, 5]
A condition-dependent index
P/F is not a fixed patient attribute. Published experimental and modeling work shows that it can change as FiO₂ changes, and it may also be affected by saturation, PEEP or CPAP, ventilation, position, and time. Compare values only with their measurement context. [3, 4]
Technical input limits
PaO₂ is limited to greater than 0 through 1000 mmHg equivalent; FiO₂ is limited to 21–100% or 0.21–1.00. These are calculator technical entry ranges, not normal values, ARDS thresholds, or clinical reference ranges. This page does not model hyperbaric context.
Altitude and treatment
Berlin and New Global Definition context at elevations above 1000 m describes raw P/F × (local barometric pressure / 760). This page has no barometric-pressure field and does not apply that definition-context adjustment, so raw P/F at altitude must not be mechanically compared with thresholds here. Recent physiologic commentary has questioned the conventional correction direction; it does not rescind or override published definitions, and this page does not calculate an alternative. The page does not determine oxygen, ventilation, proning, intubation, extracorporeal support, or other treatment. [1, 2, 7]
Adult and pediatric frameworks are distinct
The formula is pure arithmetic. The Berlin and New Global Definition paragraphs describe adult ARDS definition contexts. Children require the complete PALICC-2 PARDS framework: OI or OSI is preferred for severity in ventilated children, while P/F or S/F has a distinct PALICC-2 role with NIV or HFNC. This page does not diagnose or grade PARDS and does not automatically grade neonatal disease. [1, 2, 6]
P/F ratio and Oxygenation Index are not interchangeable
P/F divides PaO₂ by FiO₂ only. Oxygenation Index also includes mean airway pressure, so it answers a different calculation question and cannot be substituted for P/F. [5, 6]
References
- ARDS Definition Task Force. Acute respiratory distress syndrome: the Berlin Definition. JAMA. 2012;307(23):2526–2533. PMID 22797452. DOI 10.1001/jama.2012.5669.
- Matthay MA, et al. A New Global Definition of Acute Respiratory Distress Syndrome. Am J Respir Crit Care Med. 2024;209(1):37–47. PMID 37487152. PMCID PMC10870872. DOI 10.1164/rccm.202303-0558WS.
- Karbing DS, et al. Variation in the PaO₂/FiO₂ ratio with FiO₂: mathematical and experimental description, and clinical relevance. Crit Care. 2007;11(6):R118. PMID 17988390. PMCID PMC2246207. DOI 10.1186/cc6174.
- Hardavella G, et al. Oxygen devices and delivery systems. Breathe (Sheff). 2019;15(3):e108–e116. PMID 31777573. PMCID PMC6876135. DOI 10.1183/20734735.0204-2019.
- Grasselli G, et al. ESICM guidelines on acute respiratory distress syndrome: definition, phenotyping and respiratory support strategies. Intensive Care Med. 2023;49(7):727–759. PMID 37326646. PMCID PMC10354163. DOI 10.1007/s00134-023-07050-7.
- Emeriaud G, et al. Executive Summary of the Second International Guidelines for the Diagnosis and Management of Pediatric ARDS (PALICC-2). Pediatr Crit Care Med. 2023;24(2):143–168. PMID 36661420. DOI 10.1097/PCC.0000000000003147.
- Álvarez-Maldonado P, Torres-Alarcón LA, Cerón-Díaz UW. Beyond PaO₂/FiO₂ at altitude: are we correcting the ratio in the wrong direction? Ann Intensive Care. 2026;16:100087. PMID 42212004. PMCID PMC13214548. DOI 10.1016/j.aicoj.2026.100087.
- Davis MD, Walsh BK, Sittig SE, Restrepo RD. AARC clinical practice guideline: blood gas analysis and hemoximetry: 2013. Respir Care. 2013;58(10):1694–1703. PMID 23901131. DOI 10.4187/respcare.02786.
FAQ
Use oxygen partial pressure from an arterial blood-gas or equivalent arterial sample. SpO₂, venous PO₂, capillary PO₂ and an estimated PaO₂ are not direct substitutes for this calculation.
Sources: [8]
Use the unit reported with the arterial sample. The calculator explicitly converts between mmHg and kPa without rounding the internal value before calculating.
Sources: [8]
Choose percent for 40 and decimal fraction for 0.40. Those inputs are equivalent only when paired with their matching selected format.
Sources: [3]
The New Global Definition includes P/F ≤300 mmHg and permits S/F ≤315 only when SpO₂ is ≤97% in specified frameworks. Non-intubated ARDS can use P/F ≤300 with HFNO at ≥30 L/min or NIV/CPAP with at least 5 cm H₂O expiratory pressure; intubated ARDS retains separate mild, moderate, and severe numerical bands. This calculator does not calculate S/F or check the other definition requirements.
For Berlin and New Global Definition context above 1000 m, the published definition-context adjustment is raw P/F × (local barometric pressure / 760). This page does not collect local barometric pressure or apply that adjustment, so its raw result should not be mechanically compared with thresholds at altitude. A 2026 physiologic commentary questioned the conventional direction; it does not rescind or override the definitions and this page does not calculate an alternative.
For New Global Definition measurement context, obtain the paired measurement while the patient is comfortably at rest and at least 30 minutes after a change in position, FiO₂, or oxygen flow. This is a definition measurement context, not a universal wait or treatment instruction; urgent care should not be delayed.
Sources: [2]
Divide arterial PaO₂ in mmHg by FiO₂ as a decimal. For example, 80 mmHg ÷ 0.40 = 200 mmHg. Convert oxygen percentage to a decimal first; the ratio alone does not establish ARDS.
Sources: [1]
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ROX Index
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Disclaimer
Educational and informational reference only. Not intended to replace professional medical advice, diagnosis, treatment, or independent verification.