LPG composition and properties: formula, density and heat value
LPG is a mixture, mostly propane (C3H8) and butane (C4H10). It carries about 46 MJ of heat per kg, is lighter than water as a liquid, and is heavier than air as a gas. THE LPG CO. supplies certified cylinders nationwide — these properties explain why storage and ventilation rules matter.
Quick facts
Mostly propane and butane. About 46 MJ per kg. One litre of liquid becomes roughly 250 litres of gas. Gas is 1.5 to 2 times heavier than air. Burns in air at roughly 2% to 10% gas.
What LPG is made of
LPG stands for Liquefied Petroleum Gas. The base gases are propane and butane. Small amounts of propylene and butylene can be present, mostly from refinery LPG, along with traces of lighter and heavier hydrocarbons. The exact share of each depends on where it came from and the season. Compare the two on propane vs butane.
Properties table: propane, butane and typical LPG
| Property | Propane | Butane | Typical LPG mix |
|---|---|---|---|
| Formula | C3H8 | C4H10 | Mixture |
| Boiling point | About minus 42 °C | About minus 0.5 °C | Between the two, depends on the mix |
| Liquid density | About 0.50 kg/L | About 0.58 kg/L | About 0.52 to 0.55 kg/L |
| Gas density vs air | About 1.5 | About 2.0 | About 1.5 to 2.0 |
| Heat value (net) | About 46 MJ/kg | About 46 MJ/kg | About 46 MJ/kg |
| Flammable range in air | About 2% to 9.5% | About 1.8% to 8.4% | Roughly 2% to 10% |
| Vapour pressure at 20 °C | About 8 bar | About 2 bar | Between, depends on the mix |
| Colour and smell | Colourless, almost odourless | Colourless, almost odourless | Odorant added |
Why these properties matter in a Pakistani home or kitchen
- Heavier than air: a leak flows downhill and collects in low spots, drains and floors. Ventilate low, not high.
- Large expansion: about 250 times from liquid to gas, so a small leak can fill a room.
- Narrow flammable window: it burns between roughly 2% and 10% gas in air, but a leaking room passes through that range.
- Heat sensitivity: pressure rises with temperature, so keep cylinders out of the sun and away from heat — important on rooftops and in closed vans.
- Cold weather: butane-rich vapour can weaken near freezing in northern areas; protect cylinders from ice and snow.
- Odorant: a smell is added for safety, so any smell of gas should be treated seriously.
How much does a cylinder hold?
| Cylinder | Gas by weight | Approx. liquid volume at about 0.53 kg/L | Approx. heat content |
|---|---|---|---|
| Domestic | 11.8 kg | About 22 litres | About 540 MJ |
| Commercial | 45.4 kg | About 86 litres | About 2,090 MJ |
| Small portable | 5 kg | About 9.5 litres | About 230 MJ |
To estimate how long a cylinder lasts for your stove or burner, use the cylinder duration calculator. Live gas prices: LPG price in Pakistan (Rs 279.34 per kg, October 2026).
How LPG compares with other fuels
- Natural gas (Sui gas) is mostly methane, lighter than air, and has less heat per cubic metre. See LPG vs Sui gas.
- CNG is compressed methane stored at very high pressure. See LPG vs CNG.
- LNG is methane cooled into a liquid. See LPG vs LNG.
Safe handling follows from the properties
Because LPG sinks, expands and ignites in a narrow range, the safety rules are about storage, ventilation and leak checks. See the LPG safety guide and what to do if you smell gas.
Buy certified cylinders nationwide
THE LPG CO. (licence 09024041999) supplies domestic and commercial sizes through a nationwide network. See LPG cylinders, how ordering works, or order LPG. Call or WhatsApp +92 306 9701659.
Frequently asked questions
What is the chemical formula of LPG?
LPG has no single formula because it is a mixture. Its main parts are propane (C3H8) and butane (C4H10), with small amounts of propylene, butylene and other hydrocarbons depending on the source.
What is LPG made of?
Mostly propane and butane. The ratio changes with supplier and season. Domestic cylinders in Pakistan are sold as LPG, not as pure propane or pure butane.
What is the density of LPG?
As a liquid, LPG is lighter than water: roughly 0.5 to 0.58 kg per litre depending on the mix and temperature. As a gas, it is about 1.5 to 2 times heavier than air.
What is the calorific value of LPG?
About 46 MJ per kg (net) for typical LPG, close to 12.8 kWh per kg. Values vary slightly with the propane and butane share.
Why is a smell added to LPG?
Pure LPG has almost no smell. A strong-smelling chemical (a mercaptan) is added in tiny amounts so that a leak can be noticed. The smell can fade in some conditions, so do not rely on it alone.
Why is a cylinder not filled completely?
Liquid LPG expands as it warms. Cylinders are filled to a safe fraction of their volume, usually below about 85 percent, so there is room for vapour and expansion. Overfilling is dangerous.
How much gas is 1 litre of LPG?
One litre of liquid LPG becomes roughly 250 litres of gas at normal pressure. That is why a small leak can quickly fill a room.
How much heat is in an 11.8 kg domestic cylinder?
About 540 MJ of heat (at roughly 46 MJ per kg). At the October 2026 OGRA rate that gas costs about Rs 3,296 before delivery or deposit.