Pick your cylinder, read the gauge, and type the flow your prescriber set. You get the time to the red line — when to change it — and the time to empty, from the cylinder maker’s own figures.
⚠ Severe difficulty breathing, a tight chest, or blue or grey lips: call 999 (US: 911). NHS · checked 2026-09-26
Time left in this cylinder
About 1 h 45 min before the 500 psi red lineAnd about 2 h 38 min to empty — an MD (D) cylinder at 1,500 psi, 2 L/min continuous.
| Flow | To the red line | To empty |
|---|
This page does arithmetic on the cylinder maker’s figures. Your supplier’s fill, a leaking regulator, and how fast you breathe on a conserver all change the real time — so plan to the red line, carry a spare on a long trip, and never change your flow without your prescriber.
Cylinder figures checked 2026-09-26.
Quick answer: a full D cylinder (the maker calls it MD) holds 425 litres of oxygen, which is about 3½ hours at 2 litres a minute (3 h 32 min) on continuous flow; a full E holds 680 litres, about 5 hours 40 minutes. The sum is simple — gauge pressure × the tank factor ÷ your flow — but the answer to plan with is the time to the red line at 500 psi, when your supplier expects you to change it, not the time to empty.
How to use it
Pick your cylinder from the list — the size is stamped on the shoulder or printed on the label — then type what the gauge needle points at and the flow your prescriber set (the number in the regulator’s window). Leave the gauge blank for a full cylinder. If your regulator is a pulse-dose conserver, pick its ratio from the conserver’s manual; if you are not sure, leave it on Continuous, which is the shorter and safer answer.
The sum, and where the tank factor comes from
A gauge reads pressure, not oxygen. To turn pressure into litres you need the cylinder’s tank factor — how many litres each psi is worth — and that comes from two numbers its maker publishes: the oxygen it holds when full and the pressure it is full at. A D cylinder holds 425 litres at 2,015 psi, so each psi is worth about 0.21 litres. At 1,500 psi that is about 316 litres; at 2 litres a minute, 158 minutes. The 500 psi red line takes 105 litres off the top of that, which leaves about 1 hour 45 minutes of oxygen you can count on.
This page always rounds down. A calculator that tells you 2 hours when it is 1 hour 58 is one you learn not to trust in the car park.
Oxygen tank sizes
Every size below is from the cylinder maker’s own specification sheet — Catalina Cylinders, whose aluminium cylinders are the familiar green-shouldered ones. The letters are the older names: the VA’s own chart pictures its C, D and E cylinders at the heights of the M9, MD and ME. Weights are for the empty cylinder.
| Cylinder | Height | Weight (empty) | Oxygen | Full at | Full to empty at 2 L/min |
|---|---|---|---|---|---|
| M2 | 5.9 in / 15 cm | 0.75 lb / 0.34 kg | 45 L | 2,216 psi | 22 min |
| M4 | 8.7 in / 22 cm | 1.7 lb / 0.8 kg | 113 L | 2,216 psi | 56 min |
| M6 | 11.8 in / 30 cm | 2.3 lb / 1 kg | 170 L | 2,216 psi | 1 h 25 min |
| ML6 | 7.9 in / 20 cm | 2.9 lb / 1.3 kg | 170 L | 2,015 psi | 1 h 25 min |
| M7 | 9.1 in / 23 cm | 3.2 lb / 1.5 kg | 198 L | 2,015 psi | 1 h 39 min |
| M9 (C) | 10.9 in / 28 cm | 3.8 lb / 1.7 kg | 255 L | 2,015 psi | 2 h 7 min |
| MD (D) | 16.7 in / 42 cm | 5.4 lb / 2.5 kg | 425 L | 2,015 psi | 3 h 32 min |
| M22 | 16.3 in / 41 cm | 8.2 lb / 3.7 kg | 623 L | 2,216 psi | 5 h 11 min |
| ME (E) | 25.4 in / 65 cm | 7.8 lb / 3.5 kg | 680 L | 2,015 psi | 5 h 40 min |
| M60 | 23.1 in / 59 cm | 22.3 lb / 10.1 kg | 1,699 L | 2,216 psi | 14 h 9 min |
| M90 | 32.7 in / 83 cm | 30.4 lb / 13.7 kg | 2,549 L | 2,216 psi | 21 h 14 min |
| M122 | 36.2 in / 92 cm | 40.3 lb / 17.9 kg | 3,455 L | 2,216 psi | 28 h 47 min (about 1.2 days) |
| M150 | 47.2 in / 120 cm | 49.2 lb / 22.3 kg | 4,248 L | 2,015 psi | 35 h 24 min (about 1.5 days) |
Steel H and K cylinders are not on this list. We have not read a maker’s sheet for them, so we do not print numbers for them.
Why the red line matters more than empty
Home-oxygen suppliers mark the last 500 psi of the gauge in red and ask you to change the cylinder, or call them, when the needle gets there. That reserve is what covers a slow queue, a flat tyre or an appointment that overruns. So the headline answer on this page is the time to the red line, and the time to empty sits underneath it — useful to know, not something to plan a day around.
Pulse-dose conservers
A conserver sends oxygen only as you breathe in, so the same cylinder lasts several times longer than on continuous flow. How many times longer depends on the conserver and on how fast you breathe — its maker states the ratio in the manual, and it falls as your breathing speeds up. The VA’s own chart, which assumes 20 breaths a minute, works out at about three times the continuous figure. This page never assumes a ratio for you: pick yours, and the conserver answer is printed separately from the continuous one.
Why charts disagree
Put the VA’s chart, the cylinder maker’s chart and a supplier’s leaflet side by side and they will not match to the minute. Makers quote a cylinder’s capacity; suppliers fill to slightly different pressures; some charts already leave a reserve and some do not. Where the VA publishes a figure for your cylinder and flow, this page prints it next to its own, so you can see the spread rather than be surprised by it.
Worked examples
A D cylinder at 1,500 psi, 2 L/min. About 1 h 45 min to the red line and 2 h 38 min to empty.
A full E cylinder at 2 L/min. About 4 h 15 min to the red line and 5 h 40 min to empty.
A full D on a 3:1 conserver, setting 2. About 7 h 59 min to the red line — against 2 h 39 min on continuous flow.
An E cylinder reading 400 psi. Already in the red zone. Change it or call your supplier; do not start a journey on it.
FAQ
How long does a D oxygen tank last?
How long does an E oxygen tank last?
What are the sizes of oxygen tanks?
How do I calculate how long my oxygen tank will last?
When should I change my oxygen cylinder?
Can I turn my oxygen up to make the tank last differently?
What is a tank factor?
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Related reading: is an oxygen machine the same as a ventilator, does using oxygen make your lungs weaker, CPAP vs oxygen at night and how to get more oxygen at night.