Gas in a Bottle: The Physics of LPG
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Open the LPG Consumption Calculator →A portable gas cylinder for a barbecue, a heater, or a rural kitchen holds a remarkable trick of physics: a large volume of gas squeezed into a small metal bottle as a liquid. Liquefied petroleum gas — LPG — owes its usefulness to the way certain gases can be turned to liquid under modest pressure, then released as gas on demand.
What LPG Is
LPG is a mixture of light hydrocarbon gases, chiefly propane and butane, obtained from natural gas processing and oil refining. As gases they would be bulky and impractical to store, but they share a convenient property: a little pressure at ordinary temperatures turns them into liquids, packing far more fuel into a container than a gas ever could.
Liquid Under Pressure
The key is that propane and butane liquefy under moderate pressure at room temperature. Compressed into a cylinder, they exist mostly as liquid, with a layer of gas above. This is why an LPG bottle is heavy and sloshes — it holds liquid fuel — and why so much energy fits in a portable container. The pressure keeps the fuel condensed until it is needed.
| Feature | Consequence |
|---|---|
| Liquefied under pressure | Compact, portable storage |
| Boils off as gas | Ready fuel on demand |
| Sold by weight | Stored as liquid |
Boiling on Demand
When you open the valve, the pressure drops and some of the liquid instantly boils back into gas, feeding the burner — a self-refilling supply of vapor drawn from the liquid reserve below. As gas is used, more liquid boils to replace it, keeping the pressure roughly steady until the liquid runs low. The cylinder is, in effect, a small factory turning liquid into gas as fast as needed.
Freedom From the Grid
Because LPG is portable and needs no pipeline, it brought clean gas fuel to places the gas network never reached — rural homes, campsites, boats, and off-grid appliances. This portability is its great advantage. Since it is stored and sold as a liquid, its consumption is naturally measured by weight, converting a heat demand directly into kilograms of fuel drawn from the bottle.
Calculating LPG Use
To size LPG consumption, use the LPG Consumption Calculator. Compare pure propane with the Propane Consumption Calculator, and piped gas with the Natural Gas Consumption Calculator.
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