Cubic Feet of Gas Versus a Cylinder's Litres
Buy a diving or welding cylinder in the United States and it is sold by the gas it delivers: an 80, a 100, a 125. Buy the same class of cylinder in Europe and it is sold by the bottle itself: a 12-litre, a 15-litre, a 50-litre. Both numbers describe the object in your hands, yet they measure completely different things — and swapping one for the other with a plain volume conversion is where the confusion starts.
The Two Numbers on a Cylinder
Free gas volume, the US convention
Water capacity, the European convention
Service pressure links the two
Nominal ratings versus measured ones
Sizing a Cylinder Before You Buy or Fill It
Whether you are comparing two tanks in a shop listing or reading a fill-station price sheet, take one number at a time and keep track of which of the two you are holding.
Put the cubic-foot rating into the left field
Enter 80, 100, 244 — whatever the cylinder is sold as. The litre equivalent of that free gas appears at once, and a comma works as the decimal separator if you are typing something like 77,4.
Check it against the water capacity, not instead of it
The litre figure you have just produced is gas at one atmosphere. Compare it with the WC stamp on the neck only after dividing by the service pressure in bar — otherwise you are setting a bottle against the air it holds.
Reverse it for a metric cylinder
For a European bottle listed as 12 L at 232 bar, multiply first (12 × 232 ≈ 2,784 L of gas), then press the swap button (↔) so the conversion runs L → ft³ and returns roughly 98 ft³.
Copy the figure into your gas plan
The copy button on each field hands over the plain number with no unit, ready to paste into a dive plan, a consumption estimate or a purchase comparison. Ctrl + C in a field does the same.
Cylinder Ratings Side by Side
Common diving, welding, medical and industrial cylinders with both of their numbers: the gas they are sold by, and the bottle they actually are. Entries marked nominal are catalogue class names rather than measured deliveries.
| Gas cylinder | Rated content | Free gas (L) | Water capacity (L) | Service pressure |
|---|---|---|---|---|
| Aluminium 80 scuba | 77.4 ft³ | 2,191.7 | 11.1 | 3,000 psi / 207 bar |
| Steel HP100 scuba | 100 ft³ | 2,831.7 | ≈12.9 | 3,442 psi / 237 bar |
| European 12 L scuba | ≈98 ft³ (nominal) | ≈2,784 | 12.0 | 232 bar |
| Argon welding cylinder, "80 cf" | 80 ft³ (nominal) | 2,265.3 | ≈14.5 | 2,265 psi / 156 bar |
| Medical E oxygen | 24 ft³ (nominal) | ≈680 | ≈4.7 | 2,015 psi / 139 bar |
| Medical M60 oxygen | 60 ft³ | 1,699.0 | ≈11 | 2,200 psi / 152 bar |
| Medical H / K oxygen | 244 ft³ | 6,909.3 | ≈43 | 2,200 psi / 152 bar |
| Industrial 50 L, 300 bar | ≈530 ft³ (nominal) | ≈15,000 | 50.0 | 300 bar |
Multiply a water capacity by its pressure in bar and the answers come close to the rated contents, though not exactly: past roughly 150 bar a real gas stops obeying the simple proportion and packs in a little more than the ideal calculation predicts. That is why the H cylinder is sold as 244 ft³ when a straight estimate suggests nearer 232.
Handy Points for Cylinder Comparisons
Both boxes stay live while you compare
Type in either field and the other follows, so you can work down a shop's whole cylinder list without clearing or resetting anything between entries.
Metric bottle back to a US rating
The swap button flips the direction to L → ft³, which is what you want once you have turned a European cylinder's litres and bar into a free gas figure.
Cubic metres for a big industrial pack
Searchable dropdowns on both sides carry the full volume list, so a quad or a bundle quoted in m³ of gas converts as easily as a single bottle.
Figures ready for a consumption estimate
Results carry up to eight decimals and copy as a bare number, which suits a surface-air-consumption sum or a shielding-gas cost per hour of welding.
Cylinder Capacity Questions
Why is an 80 cubic foot tank only an 11 litre cylinder?
Because the two numbers measure different things. The 11.1 L is the physical space inside the aluminium bottle. The 80 ft³ — realistically about 77.4 ft³, or 2,192 L — is what the gas becomes once it escapes and expands to atmospheric pressure. At 207 bar the contents are compressed roughly two hundredfold.
What exactly is the difference between free gas volume and water capacity?
Water capacity is found by filling the empty cylinder with water and weighing it; it never changes and it appears on the neck stamp as WC. Free gas volume is derived — the water capacity multiplied by the pressure ratio — telling you what the contents would occupy at one atmosphere. One is a property of the metal, the other a property of the fill.
What does the service pressure change about a cylinder's rating?
Everything on the gas side and nothing on the bottle side. Two cylinders sharing a 12 L water capacity hold very different amounts if one is rated 200 bar and the other 300 bar — roughly 2,400 L against 3,600 L of free gas, about 85 ft³ against 127 ft³. It also explains why a low-pressure steel tank and a high-pressure one can be the same physical size yet wear different badge numbers.
How do I turn a European litre-rated cylinder into a cubic foot figure?
Two steps. First multiply the water capacity by the working pressure in bar to get litres of free gas: a 15 L bottle at 232 bar gives about 3,480 L. Then divide by 28.316846592, which lands near 123 ft³. Doing only the second step is the classic mistake — it turns a 15 L cylinder into "0.53 ft³", the size of the bottle rather than the gas in it.
Are standard cubic feet the same as actual cubic feet?
No. A standard cubic foot (scf) is a fixed quantity of gas — the amount filling one cubic foot at an agreed reference condition, commonly 60 °F and 14.73 psia in the United States. An actual cubic foot is just a cubic foot of space at whatever pressure and temperature happen to apply. Cylinder contents are quoted in scf so two bottles can be compared honestly, which is also why a cold cylinder and a warm one show different gauge pressures while holding the same amount.
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