How to Choose the Right LPG Cylinder Size

Water capacity is not the same as gas weight

Every refillable cylinder carries two different capacity figures, and confusing them is the most common sizing mistake we see in enquiries.

Water capacity is the internal volume of the shell, measured in litres. It is a fixed physical property of the cylinder and it is stamped on the collar. Gas capacity is the mass of LPG the cylinder may legally hold, measured in kilograms, and it is always lower than the water capacity figure would suggest.

The two are linked by a filling ratio, which exists because liquefied gas expands as it warms. A cylinder is never filled to its full internal volume — the remaining space, the ullage, gives the liquid room to expand without the pressure rising to an unsafe level.

Butane and propane do not fill the same

Propane has a higher vapour pressure than butane at the same temperature, so a cylinder rated for both will carry less propane by mass than butane. This is why product tables list the two separately rather than giving a single capacity.

If your market runs predominantly on propane — common where ambient temperatures drop, because butane stops vaporising readily near freezing — size against the propane figure, not the butane one. Specifying against the wrong column is a mistake that only becomes visible after the first winter.

Start from the application, not the catalogue

In practice, four questions settle the size faster than comparing datasheets:

• •Who carries it? A cylinder a single person must lift and walk with has a practical ceiling well below what the steel can handle. Domestic cooking cylinders are sized around human handling as much as around gas demand.
• •How often can it be refilled? Where the refill network is dense, smaller cylinders lower the cost of entry for the household. Where refilling means a long trip, a larger cylinder buys time and is usually worth the extra handling weight.
• •What is the burner duty cycle? A restaurant running continuous burners draws gas faster than a household, and a cylinder that is too small will frost over as the liquid struggles to vaporise at the required rate.
• •What does the existing fleet look like? Valve thread, collar design and footprint have to match the regulators, filling carousels and storage cages already in the market. A technically better cylinder that does not fit the existing infrastructure will not be adopted.

Vaporisation rate: the constraint people forget

A cylinder does not just store gas, it has to turn liquid into vapour fast enough to feed the appliance. That rate depends on the wetted surface area inside the shell and on the ambient temperature, not on how much gas remains.

The visible symptom of undersizing is frost forming on the cylinder wall and the flame dropping off even though the cylinder still has gas in it. If you are specifying for continuous commercial use, raise this with your supplier explicitly — it is a sizing question that unit capacity alone will not answer, and it is often solved by manifolding several cylinders rather than fitting one larger one.

Matching the size to the standard

Size interacts with certification. The applicable construction standard depends on the cylinder's test pressure and capacity band, and the markings a cylinder must carry change with the market it is destined for.

Decide the size and the destination market together, not in sequence. A specification that is correct for one and wrong for the other has to be re-tooled, and that is a production-slot problem rather than a paperwork problem.