About the Oxygen Tank Duration Calculator
This calculator uses the cylinder-factor method, the standard formula taught in respiratory therapy and EMS training to estimate how long a compressed oxygen cylinder will run at a given flow rate. It converts remaining gauge pressure into usable gas volume, then divides by the flow rate to get run time.
The formula
Duration in minutes equals the usable pressure (current tank pressure minus a safety reserve pressure) multiplied by the cylinder factor, divided by the flow rate:
Duration (min) = (Tank pressure − Reserve pressure) × Cylinder factor ÷ Flow rate (L/min)
The cylinder factor is a conversion constant, in liters of gas per psi of pressure, specific to the physical size of the tank. It comes from the tank's water volume and is published in reference tables for standard cylinder sizes:
- D cylinder: 0.16 L/psi
- E cylinder: 0.28 L/psi (the most common portable size)
- M cylinder: 1.56 L/psi
- G cylinder: 2.41 L/psi
- H or K cylinder: 3.14 L/psi (large stationary tanks)
Why a safety reserve?
A safety reserve pressure — commonly 200 psi — is subtracted before calculating usable duration. This margin exists because a cylinder should never be run down to zero: pressure gauges become less reliable near empty, flow can become inconsistent as pressure drops, and there is a risk of contaminants being drawn back into the regulator. The reserve pressure represents "empty" for planning purposes, even though a small amount of gas remains in the tank below that line.
Worked example
An E cylinder (factor 0.28 L/psi) reads 2000 psi and is running at 2 L/min, with a 200 psi reserve. Usable pressure is 2000 − 200 = 1800 psi. Usable volume is 1800 × 0.28 = 504 liters. Duration is 504 ÷ 2 = 252 minutes, or 4 hours 12 minutes, before the tank reaches the reserve line.
Limits of the estimate
This is a planning estimate, not a clinical guarantee. Published cylinder factors are nominal and can vary slightly between manufacturers, gauge accuracy degrades at low pressure, and actual flow delivery depends on the regulator and device in use. Always follow your facility's or device's own tank-change policy, and carry a margin beyond the calculated time for real-world use, especially during transport.