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Printed on 5/15/2026
For informational purposes only. This is not medical advice.
FiO₂ (fraction of inspired oxygen) is the concentration of oxygen in the inspired air. Room air has an FiO₂ of 21%. When supplemental oxygen is delivered via nasal cannula, each liter per minute increases the FiO₂ by approximately 4%. This tool uses the widely cited 4% per liter approximation (e.g., 2 L/min ≈ 29% FiO₂). Actual FiO₂ varies with the patient's respiratory rate, tidal volume, and degree of mouth breathing. Use the estimated FiO₂ as input for [P/F Ratio Calculator](/tools/pf-ratio) (ARDS classification) and [A-a Gradient Calculator](/tools/aa-gradient) (oxygenation deficit). For full acid-base interpretation, see [ABG Interpreter](/tools/abg-interpreter).
Formula: FiO₂ ≈ 21% + (4% × L/min) | Reverse: L/min ≈ (FiO₂% − 21) ÷ 4
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The conversion formula differs by device. Standard nasal cannula (1–6 L/min) uses the 4% rule. Simple face masks, non-rebreather masks, Venturi masks, and HFNC each have their own FiO2 ranges — see the reference table in the FAQ section.
For nasal cannula, enter the L/min setting to get the estimated FiO2%. Reference: 1 L/min ≈ 25%, 2 L/min ≈ 29%, 3 L/min ≈ 33%, 4 L/min ≈ 37%, 5 L/min ≈ 41%, 6 L/min ≈ 45%. These are estimates — actual FiO2 varies with the patient's breathing pattern.
The estimated FiO2 feeds directly into the P/F ratio (PaO2 ÷ FiO2) for ARDS classification and the A-a gradient for oxygenation assessment. Enter it as a percentage (e.g., 29) or decimal (0.29) depending on the target formula.
Intensivists, respiratory therapists
The Berlin definition of ARDS requires a P/F ratio calculated with PEEP ≥ 5 cm H₂O. FiO2 is the denominator. Accurate FiO2 estimation is essential for correct ARDS severity classification (mild 200–300, moderate 100–200, severe <100). Calculate with [P/F Ratio Calculator](/tools/pf-ratio).
Emergency physicians, nurses
Documenting oxygen requirement as FiO2% (rather than just L/min) standardizes communication across teams and allows direct comparison regardless of delivery device. Essential for triage severity scoring and handoff notes.
Hospitalists, pulmonologists
The alveolar-arterial oxygen gradient requires FiO2 as an input. An elevated A-a gradient on low FiO2 (e.g., 2 L/min nasal cannula) points to a parenchymal or vascular problem rather than hypoventilation. Use [A-a Gradient Calculator](/tools/aa-gradient).
Nurses, respiratory therapists
When upgrading from nasal cannula to a face mask or downgrading from HFNC, you need equivalent FiO2 targets. This calculator helps map nasal cannula flow rates to FiO2%, enabling better-matched device transitions.
All clinical staff
Reporting oxygen as '2 L/min via NC (FiO2 ~29%)' is more precise than flow rate alone. It communicates the patient's oxygenation support level in a universally understood format, especially for inter-hospital transfers.
The formula assumes a normal respiratory rate (12–20 breaths/min) and tidal volume (~500 mL). Tachypneic patients entrain more room air per breath, diluting the supplemental oxygen — actual FiO2 may be 5–10% lower than calculated.
Nasal cannula flow fills the nasopharyngeal reservoir between breaths. Patients who mouth-breathe primarily bypass this reservoir, reducing delivered FiO2 substantially. Consider switching to a face mask for obligate mouth-breathers.
Above 6 L/min, nasal cannula oxygen becomes uncomfortable (drying mucosa) and doesn't reliably increase FiO2 further. Upgrade to a simple face mask (6–10 L/min, ~40–60%), partial rebreather, or non-rebreather mask for higher FiO2 requirements.
High-flow nasal cannula systems deliver heated, humidified oxygen with a dialed FiO2 (21–100%) at flow rates up to 60 L/min. Unlike standard NC, the FiO2 on HFNC is set directly — the conversion formula does not apply. Read the FiO2 off the device.
Venturi (air-entrainment) masks use a fixed orifice to entrain a precise air-to-oxygen ratio. Common settings deliver 24%, 28%, 31%, 35%, 40%, or 60% FiO2 regardless of the patient's breathing pattern — ideal when a specific, reliable FiO2 is required.
If you know the target FiO2 and need to find the equivalent nasal cannula flow: L/min = (FiO2% − 21) ÷ 4. For example, target FiO2 40%: (40 − 21) ÷ 4 = 4.75 L/min ≈ 5 L/min nasal cannula.
L/min is device-specific and meaningless when the patient is switched to a different delivery system. FiO2% is universal and allows direct comparison across devices, visits, and institutions.
The 4% per liter approximation is a widely cited clinical bedside rule. Actual FiO2 varies with minute ventilation and delivery conditions. Empirical validation: Chatburn & Williams, Respiratory Care 1992; Bazuaye et al., Thorax 1992. Berlin ARDS definition: JAMA 2012;307(23):2526–2533. Venturi mask FiO2 precision: Schacter et al., Thorax 1980.
Your result shows the estimated FiO2 (fraction of inspired oxygen) corresponding to the nasal cannula flow rate entered. Room air provides an FiO2 of 21%, and each liter per minute via nasal cannula adds approximately 4%: 1 L/min ≈ 25%, 2 L/min ≈ 29%, 3 L/min ≈ 33%, 4 L/min ≈ 37%, 5 L/min ≈ 41%, 6 L/min ≈ 45%. This estimated FiO2 feeds directly into P/F ratio and A-a gradient calculations.
This is an approximation based on the "4% per liter" bedside rule. Actual FiO2 at the alveolus can vary significantly: tachypneic patients (respiratory rate > 25) entrain more room air with each breath, lowering effective FiO2 by 5–10% below the estimate. Mouth-breathing patients bypass the nasopharyngeal oxygen reservoir, further reducing delivered FiO2. For critically ill patients requiring precise FiO2, use a Venturi mask or high-flow nasal cannula system.
Use this tool when you need an FiO2 estimate for a patient on standard nasal cannula (1–6 L/min) to perform downstream calculations: P/F ratio for ARDS classification, A-a gradient for oxygenation assessment, or documentation during handoffs and transfers. It is also useful for device transition planning — when moving a patient from nasal cannula to a face mask or HFNC, knowing the approximate FiO2 helps match the new device setting to the prior oxygen requirement.
This tool is not appropriate for HFNC (where FiO2 is read directly from the device), Venturi masks (where FiO2 is labeled on the entrainment device), or non-rebreather masks (use an estimate of 60–80%). For pediatric patients, use with caution — the 4% rule is derived from adult data and is less reliable in children with different respiratory mechanics.
The 4% per liter approximation is a clinical simplification. Actual FiO2 delivery is highly variable depending on the patient's minute ventilation (respiratory rate × tidal volume). Patients with tachypnea or Kussmaul breathing entrain more room air, effectively lowering true FiO2. Conversely, patients with slow, shallow breathing may receive a higher FiO2 than estimated.
This formula applies only to standard low-flow nasal cannulas at 1–6 L/min. It does not apply to HFNC (FiO2 is dialed directly), face masks, non-rebreather masks, or Venturi masks. For critically ill patients where precise FiO2 measurement matters — such as calculating P/F ratio for ARDS classification — consider using a controlled delivery device (Venturi mask or HFNC) rather than relying on nasal cannula estimates.
Disclaimer: This tool is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider with questions about your health.
April 21, 2026 · trust-baseline
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Calculate the P/F ratio to classify ARDS severity by Berlin criteria. Mild: 200–300 (27% mortality). Moderate: 100–200 (32%). Severe: <100 (45%). Normal P/F is 400–500.
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