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Oxygen Cylinder Duration
How long the cylinder lasts at this flow, from the gauge, with the working shown.
Calculator
Cylinder
The common portable in a carry bag or on a stretcher rack. Constant 0.28 L per psi.
What the regulator's gauge shows.
Use your service’s number if it is higher.
Lasts about
45 min
44.8 minutes
Practice problems
A cylinder, a gauge reading and a flow. Answer in one go, or take the test that asks for each stage of the formula in turn.
The chapter
The method
Minutes = (gauge pressure in psi minus the safe residual) × cylinder constant ÷ flow rate in L/min
Worked example
An E cylinder reads 1,800 psi. The patient is on 10 L/min.
- Usable pressure: 1,800 − 200 = 1,600 psi.
- Litres: 1,600 × 0.28 = 448 L.
- Minutes: 448 ÷ 10 = 44.8, about 45 minutes.
Cylinder constantslitres per psi
| Cylinder | Constant | Where you meet it |
|---|---|---|
| D | 0.16 | The small portable on a first-aid kit or a stretcher. |
| E | 0.28 | The common portable in a carry bag or on a stretcher rack. |
| M | 1.56 | A larger onboard or clinic cylinder. |
| G | 2.41 | A large onboard cylinder. |
| H or K | 3.14 | The big stationary cylinder. |
Two things to remember
The calculation stops at a safe residual, 200 psi here, because a regulator cannot deliver its set flow all the way to empty and because a truly empty cylinder can draw contaminants in. Your service may set the residual higher; use its number.
Canadian gauges often read in kilopascals. 1 psi is 6.894757 kPa, so divide a kPa reading by 6.895 to get psi before using the constants, which are in litres per psi.
Estimates only. A cylinder's true contents depend on its fill, its temperature and the regulator; the safe residual your service uses may be higher than 200 psi, and a flow rate is a clinical decision from your protocols and your level, not from this page. Check the gauge, not the calculator, when it matters.
Sources
- MedicTests. Supplemental Oxygen: EMT Airway Guide, modified 1 May 2026. The duration method (usable pressure = tank pressure minus 200 psi; litres = usable pressure times the cylinder constant; minutes = litres divided by the flow rate), the cylinder constants D 0.16, E 0.28, M 1.56, G 2.41, H/K 3.14, and the note that the safe residual changes by service. Checked 2026-09-15.
- National Institute of Standards and Technology. Special Publication 811, Guide for the Use of the International System of Units (2008 edition), Appendix B conversion factors: pound-force per square inch (psi) to pascal, 6.894757 E+03, so 1 psi = 6.894757 kPa. Used for the kilopascal gauge option only. Checked 2026-09-15.
A free study aid from Delta Emergency Support Training, not certification and not medical direction. Flow rates and delivery devices are decided by your protocols and your level.