Your result shows the estimated FiO2 (fraction of inspired oxygen) that corresponds to the nasal cannula flow rate entered. Room air provides an FiO2 of 21%, and each liter per minute of supplemental oxygen via nasal cannula adds approximately 4% to this baseline. For example, 2 L/min yields an estimated FiO2 of approximately 29%, and 4 L/min yields approximately 37%. This estimated FiO2 is commonly used as an input for other clinical calculations such as the P/F ratio and A-a gradient.

Keep in mind that this is an approximation based on the widely cited "4% per liter" rule. The actual FiO2 delivered to the patient's alveoli can vary significantly depending on their breathing pattern, respiratory rate, tidal volume, and whether they are mouth-breathing. Patients with rapid, shallow breathing will dilute the supplemental oxygen more, resulting in a lower effective FiO2 than estimated.

Use this conversion tool whenever you need an approximate FiO2 value for a patient receiving oxygen via standard nasal cannula at flow rates of 1–6 L/min. The most common clinical scenario is when you need the FiO2 to calculate a P/F ratio (PaO2/FiO2) for ARDS classification or to compute an A-a gradient — both of which require FiO2 as an input.

This tool is also useful in clinical documentation, when transitioning patients between oxygen delivery devices, or when communicating oxygen requirements during handoffs and transfers. It provides a quick reference that avoids the need to memorize the conversion table.

The 4% per liter approximation is a clinical simplification. Actual FiO2 delivery via nasal cannula is highly variable and depends on the patient's minute ventilation (respiratory rate multiplied by tidal volume). Patients with high minute ventilation (such as those with tachypnea or Kussmaul breathing) entrain more room air with each breath, effectively diluting the supplemental oxygen and lowering the true FiO2. Conversely, patients with low respiratory rates may receive a higher FiO2 than estimated.

This approximation applies only to standard low-flow nasal cannulas (typically 1–6 L/min). It does not apply to high-flow nasal cannula (HFNC) systems, which deliver heated humidified oxygen at rates up to 60 L/min with precise FiO2 control. It also does not apply to face masks, non-rebreather masks, or Venturi masks, each of which has its own FiO2 delivery characteristics. For critically ill patients or those requiring precise FiO2, direct measurement or a controlled delivery device is preferred.