This oxygen tank duration calculator applies one published relation and nothing else: usable pressure in a compressed-gas cylinder, multiplied by that cylinder's tank factor, divided by the continuous flow rate. It returns a duration. It does not tell you whether that duration is enough for a journey, an appointment or a power cut, and is not designed to. Running out of medical oxygen is an emergency for a person who depends on it, so this page reports a number and stops there.
Arb Digital builds these free calculators to do one job accurately and to be honest about the edge of what they know. On a page like this one, the second half matters more. The arithmetic is simple enough that a respiratory therapist does it in their head. What it cannot see is the regulator, the temperature, a leak at the connection, the accuracy of the gauge, or the fact that a cylinder's last few hundred psi do not deliver a stable flow. Those decide whether a real cylinder lasts as long as a formula says.
What This Oxygen Tank Duration Calculator Does
You enter four things: gauge pressure, the reserve pressure below which the cylinder is treated as empty, the tank factor for the cylinder size, and the continuous flow rate. The calculator subtracts the reserve to get a usable pressure, multiplies by the tank factor for a usable volume in litres, and divides by the flow rate for minutes. It shows every intermediate figure so you can check each step rather than trusting one headline.
It produces no verdict. There is no green box, no "enough for your trip", no comparison against a journey length. A page that told a reader their supply was sufficient would be making a clinical judgement on incomplete information, and the cost of getting that wrong is measured in harm rather than in a bad estimate. The number is the output; the decision belongs to the person and their care team.
How to Use It
- Pick the cylinder size, then check the factor. The dropdown fills a commonly published tank factor. Compare it against your own cylinder label before relying on it, and overwrite it if they differ.
- Enter the gauge pressure. Read it with the cylinder open and settled, at the temperature the cylinder will actually be used at.
- Enter the reserve pressure your service uses. The default of 200 psi is the figure most often published, but a transport service or a home-oxygen provider may require a much larger margin. Use theirs.
- Enter the continuous flow rate. The prescribed litres per minute, not the maximum the regulator can deliver. If the device is a pulse-dose conserver, this relation does not apply — see the section below.
- Read the duration and the three supporting numbers, then take them to the person who is responsible for your oxygen supply. That is the step this page cannot do for you.
The Formula and How It Is Calculated
The relation is: duration in minutes = (gauge pressure − reserve pressure) × tank factor ÷ flow rate. The tank factor is simply the cylinder's rated gas volume divided by its rated service pressure, which is why it has units of litres per psi. An E cylinder holding roughly 660 litres at 2,200 psi gives 660 ÷ 2,200 = 0.30 litres per psi, which is where the commonly quoted E factor comes from.
A published worked example makes it concrete. The review Essentials of Airway Management, Oxygenation, and Ventilation: Part 1 gives the equation as (psi × F) ÷ L/min, with F approximately 0.3 for E cylinders and approximately 3.0 for large H cylinders, and works the case of an E cylinder at 500 psi running at 15 litres per minute. That comes to (500 × 0.3) ÷ 15 = 10 minutes. Enter a factor of 0.3, a gauge pressure of 500, a reserve of 0 and a flow of 15 in the fields above and you will get the same ten minutes back. Note what the published example assumes: no reserve at all. Set a realistic reserve and the same cylinder gives you considerably less.
The StatPearls chapter Gas Cylinders, published on the NCBI Bookshelf, sets out the same idea from the other direction, using Boyle's law: measured pressure divided by the 2,200 psi fill pressure, multiplied by the E cylinder's 660 litre capacity, gives the litres remaining, which you then divide by the flow setting. It is the identical calculation with the division done in a different order.
Where the Published Tank Factors Come From, and Where They Disagree
The factors in the dropdown — D 0.16, E 0.28, M 1.56, G 2.41 and H/K 3.14 litres per psi — are the set carried in standard respiratory-care references. They are not a single legally defined standard, and a page presenting them as one would be quietly misleading.
Compare them with the capacities the StatPearls Gas Cylinders chapter publishes at 2,200 psig: D 425 L, E 660 L, G 3,400 L and M 3,450 L. Divide each by 2,200 and E gives 0.30, close to the conventional 0.28. But D gives 0.19 against a conventional 0.16, and G gives 1.55 against a conventional 2.41 — a gap of more than fifty per cent for the same letter. Letters are not globally standardised, and rated fill pressures differ too. The paper How many oxygen cylinders do you need to take on transport? A nomogram for cylinder size and duration works from a UK set entirely — D at 340 litres, CD at 460 litres, F at 1,360 litres — so a reader assuming a US D cylinder while holding a UK one is a quarter out before starting.
This is why the tank factor on this page is an editable input rather than a locked lookup. The authoritative figure for your cylinder is the one printed on the cylinder or supplied by the company that fills it.
What This Calculation Ignores
The relation assumes an idealised cylinder emptying at a constant rate into a perfect delivery system. Every one of the following pushes the true duration away from the calculated one, almost always downwards.
- Regulator behaviour. Regulators bleed, flowmeters are calibrated at a reference pressure, and a device set to "2" may not deliver exactly two litres per minute across the whole pressure range.
- Pulse-dose versus continuous flow. A conserver delivers a bolus on inhalation, so consumption depends on respiratory rate and breath detection. Its litres-per-minute setting is a nominal equivalence, not a flow, and this formula does not model it.
- Temperature. Gas pressure falls as a cylinder cools, so one gauged in a warm room reads lower once carried into a cold car.
- Leaks. A slow leak at the yoke, the regulator or a loose connection consumes gas that never reaches the patient and is invisible to the arithmetic.
- Gauge accuracy. Mechanical gauges have a tolerance, and it is usually widest at the ends of the scale — which is exactly where a nearly empty cylinder sits.
- The unusable tail. The last portion of a cylinder does not deliver a stable flow, because pressure falls below what the regulator needs to hold the set rate. This is why a reserve pressure exists at all.
Why the Reserve Pressure Is Not Optional
It is tempting to set the reserve to zero and read the biggest number the page will give. The reserve exists precisely because that number is fiction at the bottom of the range. A cylinder is treated as empty well above zero psi so there is a documented margin between "the gauge is getting low" and "there is no gas", and so the changeover happens while the delivery system still works properly rather than during a fade.
Two hundred psi is the figure most commonly published as that changeover point, and it is the default here for that reason. It is not a universal rule. Ambulance and retrieval services frequently require far larger margins, often a multiple of the calculated journey requirement rather than a fixed pressure. If your service publishes a rule, that rule — not this page's default — applies.
Units, Bar and Litres per Minute
The tank-factor convention here is expressed in psi, because that is how the sources cited publish it. Much of the world gauges medical gas in bar instead. One bar is roughly 14.5 psi, so 137 bar is about 1,990 psi — but converting the pressure is only half the job, since a factor in litres per psi cannot be used with a bar reading. Working in bar, the cleaner route is the rated volume directly: litres remaining equals rated litres times gauge bar divided by rated bar. Our general unit converter and pressure calculator handle the pressure conversion itself, and the Boyle's law calculator and ideal gas law calculator cover the underlying gas relations if you want to see why pressure and volume trade off the way they do.
Arb Digital builds content and tools for regulated and health-adjacent businesses, where clear sourcing, honest limits and defensible claims matter more than clever copy. Tell us what you are publishing and we will tell you how we would frame it.
Browse Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Using a tank factor from the internet instead of the cylinder. A D cylinder in one country is not a D cylinder in another, and the factor can differ by a quarter or more.
- Setting the reserve to zero. It inflates the answer by exactly the portion of the cylinder that is least reliable.
- Applying continuous-flow arithmetic to a pulse-dose conserver. The two consume gas in completely different ways.
- Treating one gauge reading as final. Pressure moves with temperature, so an indoor reading is not an outdoor one.
- Treating a duration as a permission. A calculated time is not clearance to travel or to skip a spare cylinder. That sits with a clinician and the oxygen supplier.
Related Free Tools From Arb Digital
If you came here for the gas physics rather than the clinical planning, the Boyle's law calculator and the ideal gas law calculator show how pressure, volume and temperature trade against each other, and the air density calculator covers the related density question. For flow problems outside medicine, the flow rate calculator handles volume over time in general terms, and the time duration calculator converts an awkward minutes figure into hours and minutes. The full free online tools hub lists everything else.
Frequently Asked Questions
With a tank factor of 0.28, a gauge reading of 2,000 psi and a 200 psi reserve, the arithmetic gives (2000 minus 200) times 0.28 divided by 2, which is 252 minutes or 4 hours 12 minutes. That is a planning estimate under ideal assumptions, not a supply guarantee, and it changes with the factor your cylinder actually carries.
It is the cylinder's rated gas volume divided by its rated service pressure, expressed in litres of gas per psi. An E cylinder rated at 660 litres and 2,200 psi gives 660 divided by 2,200, or about 0.30 litres per psi. Multiplying a usable pressure by this factor converts pressure into litres.
It is the figure most commonly published as the point at which a cylinder is treated as empty and changed, because below roughly that pressure the regulator can no longer hold a stable flow and gauge readings become least reliable. Many services set a larger margin, and their rule takes precedence over any default.
No. The relation assumes continuous flow. A conserving device delivers a bolus of oxygen on inhalation, so its consumption depends on breathing rate and on how the device detects a breath. The number on a conserver dial is a nominal equivalence rather than a litres-per-minute flow, and this formula does not describe it.
Common reasons include a leak at the regulator or connection, a colder environment lowering the gauge pressure, gauge tolerance at the low end of the scale, a flow rate that was increased during use, and the fact that the last portion of a cylinder does not deliver a stable flow. The arithmetic sees none of these.
No. This page reports a duration and nothing more. How much oxygen to carry, what reserve to hold and whether to take a spare cylinder are decisions for a clinician and your oxygen supplier, who know your prescription, your equipment and the rules that apply to your service.
One bar is approximately 14.5 psi, so you can convert the pressure first. A cleaner route when working in bar is to skip the tank factor entirely and use the cylinder's rated volume: litres remaining equals rated litres multiplied by gauge bar divided by rated bar, then divide by the flow rate.
No. Cylinder letters are not globally standardised, rated fill pressures differ between suppliers and countries, and published capacity tables disagree for some sizes by a wide margin. Always take the figure from the cylinder label or from the company that fills it.
This tool produces a planning estimate only and gives no verdict on whether a supply is adequate. It must never be the sole basis for deciding how much oxygen to carry or how long a cylinder will support you. Your clinician and your oxygen supplier set the real safety margin, and you should carry a documented reserve at all times. If your oxygen supply is at risk of running out, or if you become breathless or unwell, call your emergency services immediately.
