Oxygen Therapy Calculator
O2 flow rate to FiO2 conversion and cylinder duration estimation.
About this calculator
Low-flow oxygen delivery devices don't deliver a fixed, precise FiO2 -- the actual fraction of inspired oxygen depends on how much room air a patient entrains around the device, which varies with the patient's own breathing pattern. This calculator uses commonly cited rule-of-thumb approximations rather than a single universal formula: a nasal cannula adds roughly 4 percentage points of FiO2 per liter per minute of flow up to about 6 L/min (room air's 21% plus roughly 4% per liter, capping near 44-45%), a simple face mask run at 5-10 L/min delivers roughly 40-70%, and a non-rebreather mask -- which requires at least 10 L/min to keep its reservoir bag inflated -- delivers roughly 60% at that minimum flow, rising toward nearly 100% by 15 L/min. Because the three devices have such different output ranges, which device is selected changes the delivered FiO2 far more than fine-tuning the exact flow rate within a device's typical range does.
Cylinder duration is estimated from the tank's remaining pressure, a size-specific conversion factor, and the current flow rate, and is meant for planning ahead (arranging a refill or backup tank) rather than as a precise real-time gauge -- actual duration in practice is affected by valve type and regulator behavior this simplified estimate doesn't model. The target SpO2 input feeds the Approx Min Flow figure, but at the default and most typical flow rates the two calculation paths land on the same number, so in practice adjusting target SpO2 usually doesn't change any displayed result.
Medical Disclaimer
This calculator is for informational and educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making decisions about your health. Never disregard professional medical advice or delay seeking it because of results from this tool.
Inputs
Results
Estimated FiO2 (%)
29%
How to Use This Calculator
- Enter the O2 flow rate (L/min) and select the delivery device (nasal cannula, simple mask, or NRB).
- Select cylinder size and enter current cylinder pressure (psi).
- Set target SpO2 (%) to confirm the prescribed flow rate should achieve your oxygenation goal.
- Review estimated FiO2, cylinder duration in minutes and hours.
- Plan oxygen refills based on duration -- keep a backup cylinder when duration is less than 8 hours.
How the result changes with O2 Flow Rate (L/min)
| O2 Flow Rate (L/min) | Estimated FiO2 (%) |
|---|---|
| 1 | 25% |
| 1.5 | 27% |
| 3 | 33% |
| 5 | 41% |
What each input means
- O2 Flow Rate (L/min)
- Oxygen flow rate in liters per minute.
- Delivery Device
- Select oxygen delivery device.
- Cylinder Pressure
- Remaining pressure in the oxygen cylinder (psi).
- Cylinder Size
- Select oxygen cylinder size.
- Target SpO2 (%)
- Desired oxygen saturation target (typically 88-92% for COPD, 94-98% otherwise).
What each result means
- Estimated FiO2 (%)
- Approximate fraction of inspired oxygen delivered by the device at this flow rate.
- Cylinder Duration (min)
- Estimated time until cylinder empties at current flow rate.
- Cylinder Duration (hrs)
- Cylinder duration in hours.
- Estimated PAO2 (mmHg)
- Estimated alveolar oxygen partial pressure from the alveolar gas equation (sea level, PaCO2=40).
- Flow Rate (L/min)
- Current oxygen flow rate.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersO2 Flow Rate (L/min) = 2, Delivery Device = 0, Cylinder Pressure (psi) = 2000, Cylinder Size = 0 = 5 input(s) provided
- Calculate Estimated FiO229 = 29
- Calculate Cylinder Duration280 = 280
- Calculate Cylinder DurationCylinder Duration = durationMin / 604.7 = 4.7
Engine last updated . Checked against 1 independently-derived test — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Does switching oxygen delivery devices matter more than adjusting the flow rate?
Generally yes -- the three common low-flow devices modeled here (nasal cannula, simple face mask, non-rebreather) cover very different FiO2 ranges, from roughly 24-45% for a nasal cannula up to nearly 100% for a non-rebreather at high flow. Fine-tuning the exact flow rate within a single device's typical range moves the delivered FiO2 by a comparatively small amount next to the difference between devices themselves.
Why does a non-rebreather mask need a minimum flow rate of 10 L/min?
Below roughly 10 L/min, the mask's reservoir bag can't stay adequately inflated between breaths, and if the bag collapses the patient starts rebreathing exhaled air instead of drawing from the oxygen-filled reservoir. That's why non-rebreather flow is conventionally kept at 10-15 L/min rather than run lower the way a nasal cannula or simple mask can be.
Is the estimated FiO2 from a nasal cannula or mask an exact number?
No -- these are approximations, not measured values. The actual fraction of inspired oxygen a patient receives from a low-flow device depends on their own breathing pattern (tidal volume, respiratory rate, and how much room air they entrain around the device), which this calculator cannot observe. The commonly cited rule-of-thumb figures used here are a reasonable planning estimate, not a substitute for a measured SpO2 or blood gas in a specific patient.
How is cylinder duration estimated, and what does it not account for?
It divides the cylinder's remaining pressure (in psi) by the current flow rate, scaled by a size-specific conversion factor for the selected tank. This is a planning estimate meant to flag when a refill or backup cylinder is needed soon, not a precise real-time gauge -- actual duration can differ based on regulator behavior and the specific cylinder's true fill characteristics.
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