How this instrument works
A compressed-gas oxygen cylinder's gauge pressure drops roughly in proportion to how much oxygen is left inside it, since the gas is compressed into a fixed physical volume. Multiplying that gauge pressure (in psig) by a cylinder-specific 'tank factor' — a constant that folds in the cylinder's internal volume — converts the pressure reading into an estimated volume of oxygen remaining, in liters. Dividing that by the flow rate in liters per minute then gives an estimate of how many minutes of supply are left at that flow rate.
The tank factor depends entirely on the physical size of the cylinder, since a bigger cylinder holds more actual gas volume at the same fill pressure. This instrument uses four commonly referenced tank factors from respiratory-therapy training materials: 0.16 for a D cylinder, 0.28 for an E cylinder, 1.56 for an M cylinder, and 3.14 for an H or K cylinder — larger cylinders carrying proportionally larger factors, reflecting their greater internal volume.
This is a planning estimate, not a substitute for a cylinder's own pressure gauge and flow regulator, and it shouldn't be treated as the final word on exactly when a tank will run dry. Medical oxygen cylinders should be treated as depleted once pressure drops to a documented safe residual pressure — commonly cited around 200 psig for medical cylinders — well before the gauge reaches zero, since flow accuracy and delivery reliability degrade as pressure runs low. Always follow the specific guidance, safe-residual-pressure figure, and clinical protocol that apply to your own equipment rather than relying solely on this estimate for a real patient-care or emergency situation.
This kind of duration estimate is routine work for EMS crews checking a portable cylinder before a transport, respiratory therapists planning how many cylinders to send home with a patient, and home-oxygen users figuring out whether a tank will last through a trip away from their stationary concentrator.
- Select Cylinder size — D, E, M, or H/K — each with its own built-in tank factor.
- Enter Tank pressure (psig) — the current reading on the cylinder's gauge.
- Enter Flow rate (L/min) — the prescribed or planned oxygen flow.
- Read Duration (minutes) and Duration (hours) for the estimated remaining supply time.
- Recalculate using a lower pressure figure than the true gauge reading if you want to build in a safety margin above your cylinder's stated safe residual pressure.
Worked example — a full E-cylinder at a 2 L/min flow
A home-oxygen patient checks a full E-cylinder, tank factor 0.28, reading 2,000 psig on the gauge, before heading out for the day at their prescribed 2 L/min flow rate. Duration (minutes) = 0.28 x 2,000 / 2 = 560 / 2 = 280.0 minutes.
Duration (hours) = 280.0 / 60 = 4.667 hours — a little under 4 hours and 40 minutes of supply at that flow rate, a standard-enough figure that it's commonly used as a reference worked example in respiratory-therapy training materials. In practice, the patient would want to plan around a shorter usable window than the full 4.667 hours, stopping well before the gauge reaches the cylinder's documented safe residual pressure rather than running it down toward zero.
Questions
How do you calculate how long an oxygen tank will last?
Multiply the cylinder's tank factor (a constant based on its physical size) by the current gauge pressure in psig, then divide by the oxygen flow rate in liters per minute — the result is the estimated number of minutes of supply remaining. For a full E-cylinder (factor 0.28) at 2,000 psig running a 2 L/min flow, that's 0.28 x 2,000 / 2 = 280 minutes, or about 4 hours 40 minutes.
What is a "tank factor" and where do those numbers come from?
A tank factor is a constant, specific to each physical cylinder size, that converts a pressure reading into an estimated volume of oxygen — larger cylinders hold more gas at the same fill pressure, so they carry proportionally larger factors. This instrument uses four commonly cited factors from respiratory-therapy training references: 0.16 for D, 0.28 for E, 1.56 for M, and 3.14 for H/K cylinders.
Should I run a cylinder all the way down to 0 psig?
No. Oxygen cylinders are generally treated as depleted once pressure drops to a manufacturer- or protocol-defined safe residual pressure, commonly cited around 200 psig for medical cylinders, well before the gauge actually hits zero — flow accuracy and delivery reliability from a regulator both degrade as pressure runs very low. Always follow your specific equipment's documentation and applicable clinical protocol for the exact residual-pressure figure that applies, rather than planning to use every last psig.
Why do different cylinder sizes have such different tank factors?
Because tank factor reflects the cylinder's physical internal volume. A larger cylinder, like an H or K (factor 3.14) compared to a small D cylinder (factor 0.16), physically holds far more compressed gas at the same fill pressure, so the same psig reading corresponds to a much larger volume of oxygen — and therefore a proportionally longer duration at any given flow rate.
Is this calculator a substitute for my oxygen equipment's own gauge or a clinician's guidance?
No — it's a planning estimate for questions like how many cylinders to pack for a trip or transport, not a substitute for reading your equipment's actual pressure gauge in real time or following your prescribed clinical protocol. Always treat your equipment's live gauge reading, the manufacturer's documented safe residual pressure, and your care team's guidance as authoritative over any estimate produced here.