How long will the tank last?
Anyone using portable oxygen, at home, in an ambulance or during a hospital transfer, needs to know when the cylinder will run out. The answer depends on three things: how much oxygen the cylinder holds, how much pressure is left and how fast it flows.
The formula
Duration (minutes) = (gauge pressure − safe residual pressure) × tank factor ÷ flow (L/min)
The tank factor converts pressure into liters of oxygen for each cylinder size:
| Cylinder | Factor (L/psi) | Approximate full volume |
|---|---|---|
| D | 0.16 | 350–425 L |
| E | 0.28 | 600–680 L |
| M | 1.56 | about 3,000 L |
| G | 2.41 | about 5,300 L |
| H / K | 3.14 | about 6,900 L |
Full cylinders are usually at about 2,000 to 2,200 psi.
Worked example
An E cylinder shows 2,000 psi. You keep a 200 psi reserve, and the patient is on 2 L/min.
- Usable oxygen = (2,000 − 200) × 0.28 = 504 L
- Duration = 504 ÷ 2 = 252 minutes, or 4 hours 12 minutes
At 4 L/min, the same tank lasts only 2 hours 6 minutes. An M cylinder at the same pressure and 3 L/min lasts about 15 hours 36 minutes.
Why keep a reserve?
Regulators deliver less accurate flow at low pressure, and a reserve gives time to swap cylinders safely. 200 psi is the most common safety margin; some services use 500 psi for transport, where delays are unpredictable.
Practical tips
- Check the gauge before every trip and bring a spare for any outing longer than half the calculated duration.
- Conservers (pulse-dose devices) release oxygen only when you breathe in and can extend duration two- to four-fold; their settings are not equivalent to continuous L/min, so use the manufacturer’s chart.
- Store cylinders upright or secured, away from heat, and never near open flames or smoking. Oxygen makes fires burn far more intensely.
- Never use oil or grease on regulators or fittings.
- On airplanes, compressed oxygen cylinders are generally not allowed in the cabin; most airlines accept FAA-approved portable oxygen concentrators instead.
Limits
The formula ignores small effects such as temperature and gas compressibility, and gauges are not perfectly precise. Treat the result as an estimate and keep a margin.
This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.