About the Oxygen Tank Duration Calculator
Knowing how long an oxygen cylinder will last is essential for patient transport, home oxygen users planning an outing, and anyone checking stock before a shift. This calculator uses the standard cylinder-factor method taught in EMS, respiratory therapy and nursing: usable pressure multiplied by the cylinder’s litres-per-psi constant, divided by the flow rate.
Choose a US cylinder size (D, E, M, G or H/K) or enter your own cylinder factor, then enter the current gauge pressure, a safe residual pressure to keep in reserve (200 psi is the usual convention) and the flow rate in litres per minute. A metric mode does the same calculation in bar using the cylinder’s water capacity in litres, as printed on European and many other cylinders.
The result is the time until the tank reaches the safe residual, assuming a continuous, constant flow. Pulse-dose conservers, leaks and temperature changes alter the real duration, so always carry a margin. The flow rate itself must be the one prescribed for the patient — this tool does not recommend oxygen settings.
With the default inputs, the duration until safe residual is 4h 12m. Change any value above to recalculate instantly.
How to use the oxygen tank duration calculator
- 1Choose US (psi) or metric (bar) units to match the regulator gauge.
- 2Select the cylinder size, or enter a custom factor or the water capacity.
- 3Read the gauge and enter the current pressure.
- 4Keep the default safe residual or enter your service’s policy.
- 5Enter the prescribed flow rate and read the duration.
Formula and method
A cylinder factor converts gauge pressure into litres of gas: it is the cylinder’s full capacity in litres divided by its full service pressure. The commonly used US constants are D 0.16, E 0.28, M 1.56, G 2.41 and H/K 3.14 litres per psi (for example an H cylinder holds about 6,900 L at 2,200 psi). Subtracting a safe residual — typically 200 psi — leaves a reserve so the tank is changed before it runs dry.
In metric mode the litres of oxygen are the usable pressure in bar multiplied by the cylinder’s water (internal) capacity in litres, because compressed gas volume scales almost linearly with pressure. Dividing by a continuous flow rate gives minutes. Pulse-dose conservers deliver oxygen only on inhalation and can extend duration several-fold.
- Gauge pressure
- Current cylinder pressure (psi or bar)
- Safe residual
- Pressure kept in reserve, commonly 200 psi
- Cylinder factor
- Litres per psi for the cylinder size
- Flow
- Prescribed continuous flow in litres per minute
Worked examples
E cylinder at 2,000 psi, 2 L/min
(2,000 − 200) × 0.28 = 504 litres usable. At 2 L/min that lasts 504 ÷ 2 = 252 minutes, or 4 hours 12 minutes.
Full M cylinder at 3 L/min
(2,200 − 200) × 1.56 = 3,120 litres. Divided by 3 L/min this gives 1,040 minutes — about 17 hours 20 minutes.
Half-empty D cylinder at 4 L/min for transport
(1,500 − 200) × 0.16 = 208 litres; at 4 L/min that is only 52 minutes, so a longer transfer would need a second cylinder.
Metric 10 L cylinder at 200 bar, 5 L/min
A 10-litre water-capacity cylinder at 200 bar with a 10 bar reserve holds (200 − 10) × 10 = 1,900 litres usable, which lasts 380 minutes at 5 L/min.
Frequently asked questions
How long does an E oxygen tank last?+
A full E cylinder (about 2,000–2,200 psi) holds roughly 500–560 usable litres after a 200 psi reserve, so it lasts about 4.2–4.7 hours at 2 L/min, a little over 2 hours at 4 L/min and under an hour at 10 L/min.
What is a cylinder factor?+
It is the number of litres of oxygen released per psi of pressure drop for a particular cylinder size, calculated as full capacity in litres divided by full service pressure. The E cylinder factor is 0.28 L/psi.
Why subtract a safe residual pressure?+
Keeping a reserve — usually 200 psi in EMS and home use, while some anaesthesia references use 500 psi — means the cylinder is replaced before it runs out completely, prevents moisture and contaminants entering an empty tank and allows for gauge inaccuracy.
Does a pulse-dose conserver change the duration?+
Yes. Conservers deliver oxygen only when you breathe in, so a cylinder may last two to four times longer than at the same continuous flow. Use the manufacturer’s duration chart for conserving devices.
What flow rate should I use?+
Use only the flow rate prescribed by the patient’s clinician. This calculator estimates cylinder duration for a given flow; it does not recommend oxygen therapy settings.
For healthcare professionals and education only. This calculator implements a published formula or score but is not a diagnosis and does not replace clinical judgment. Verify inputs and results against the original source before using them in patient care.
Sources
- Das S, Chattopadhyay S, Bose P. The anaesthesia gas supply system. Indian J Anaesth 2013;57(5):489-99 (E-cylinder tank factor 0.28 L/psi and duration formula)
- Inkrott JC, White JR. Oxygen Economics: The Use of Heated High-Flow Nasal Oxygen in Air Medical Transport of the Adult Patient. Air Med J 2021 (cylinder-factor method, D cylinder)