A cylinder holds a fixed internal volume of gas at high pressure. Expanding that gas to atmospheric pressure gives the usable volume, which divided by the flow rate gives the run time.

V_cyl is the water capacity of the cylinder in litres, meaning its internal volume, which is the figure stamped on the shoulder and the way cylinder sizes are quoted. p_abs,min is the minimum usable pressure you entered, expressed in absolute terms. Q is the flow rate. V_usable comes out in litres at 1.01325 bar and at the temperature of the cylinder.

Compressibility

The formula above assumes the gas obeys the ideal gas law, which stops being true at cylinder pressures. Z corrects for it. The direction depends on the gas: above 1 for hydrogen and nitrogen at typical fill pressures, meaning the cylinder holds less than the ideal calculation suggests, and below 1 for oxygen, argon and methane, meaning it holds more. The size of the correction is a few percent for most permanent gases and around 20 % for methane at 200 bar (Peng-Robinson estimate, 2026-08-09, indicative only). For a number you can rely on, take Z from your gas supplier or from the NIST Chemistry WebBook at your fill pressure and temperature.

What this does not account for

Temperature is assumed constant. A cylinder stored in the cold holds proportionally more gas than the same pressure reading at room temperature, and the pressure reading itself moves with temperature. Flow given in sccm or slm carries its own reference conditions, usually 0 or 20 °C, which can differ from the cylinder temperature by several percent. Purge losses, regulator bleed and leaks are not included, and in intermittent use they can dominate everything above. Treat the result as a planning figure, not as a reason to run a cylinder to the last litre.