Oxygen cylinder duration calculator (tank factor)
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Result
The time is rounded down to whole minutes. For exam practice only. This is a study aid, never a clinical decision: always follow the prescription, local protocols and the product information.
Work out how long an oxygen cylinder will last at a given flow rate. For US cylinders, choose the size (D, E, M, G or H) and enter the gauge pressure in psi; the calculator uses the standard tank factor and keeps a safe residual pressure in reserve. For metric cylinders, enter the water capacity in litres and the pressure in bar.
Step-by-step solution
Usable pressure = gauge pressure − safe residual
Usable oxygen = usable pressure × cylinder factor
Duration = usable oxygen ÷ flow rate
Key formulas
- Cylinder duration
- min = (psi − 200) × factor / (L/min); D 0.16, E 0.28, H 3.14 L/psi
- Nasal cannula
- FiO₂ ≈ 21% + 4% × L/min (1–6 L/min)
How to calculate how long an oxygen cylinder will last
- Subtract the safe residual pressure from the gauge pressure: 2000 − 200 = 1800 psi usable.
- Multiply by the tank factor (L/psi): D 0.16, E 0.28, M 1.56, G 2.41, H or K 3.14. For an E cylinder: 1800 × 0.28 = 504 L.
- Divide by the flow rate: 504 L ÷ 2 L/min = 252 min, about 4 h 12 min.
- Metric cylinders: usable litres = (pressure in bar − residual) × water capacity in L, then divide by the flow.
Questions and answers
Where do the tank factors come from?
A tank factor is the cylinder volume in litres divided by its full pressure in psi. An E cylinder holds about 625 L at about 2200 psi, which gives 0.28 L/psi; a D cylinder holds about 350 L (0.16 L/psi) and an H cylinder about 6900 L (3.14 L/psi).
What is the safe residual pressure?
A reserve that is not counted as usable, so the cylinder is changed before it runs empty. 200 psi is common in US teaching; follow the local policy, which may use another value or a coloured zone on the gauge.
Why round the time down?
Running out of oxygen early is the dangerous error, so the duration is rounded down to whole minutes.
Does the formula work for liquid oxygen?
No. Liquid oxygen systems are measured by weight or by a contents gauge; this calculation is for compressed gas cylinders.
Practise it yourself
Practise Oxygen therapy & respiratory calculations
A calculator gives the answer. In the exam you need the method: practise it with new problems, hints and worked solutions.