Chemical composition
Propane's molecular formula is C3H8 — three carbon atoms bonded with eight hydrogen atoms. At ambient temperature and atmospheric pressure it's a colorless, odorless gas. Under ~120 psi pressure at 70°F, propane liquefies, which is why it can be stored efficiently in pressurized tanks at 1/270th the volume of vapor.
Where it comes from
Two sources combined produce US propane: roughly two-thirds from natural gas processing (separated from methane at fractionation plants) and one-third from crude oil refining (collected during the refining of heavier petroleum products). Propane is a by-product — refineries and gas processors produce it whether or not demand exists, which shapes US propane economics.
Key properties
- BTU content: ~91,500 BTU per liquid gallon
- Boiling point: -44°F at atmospheric pressure
- Vapor density: 1.5× heavier than air (so leaks pool low)
- Storage pressure: ~120 psi at 70°F in a residential tank
- Combustion ratio: ignites in air at 2.1–9.5% concentration
- Odorant: ethyl mercaptan added at the wholesale level (rotten-egg smell)
What propane is used for
US residential applications: space heating, water heating, cooking, clothes drying, backup generators, fireplaces, pool heaters, and outdoor grills. Commercial: forklifts, school buses, food trucks, restaurant cooking. Agricultural: grain drying, livestock building heat, irrigation engines. See propane usage for the full breakdown.
FAQ
Is propane the same as LPG?
LPG (liquefied petroleum gas) is a category that includes propane and butane. In US residential context, 'propane' and 'LPG' are used interchangeably — both refer to the C3H8 propane molecule. See what is LP gas and propane and butane.
Is propane natural gas?
No. Natural gas is primarily methane (CH4), distributed through utility pipelines at low pressure. Propane (C3H8) is delivered by truck to onsite tanks and stored under pressure. See propane vs natural gas.