Liquid propane (residential tank)
- 1 gallon liquid propane = 91,500 BTU = ~26.8 kWh = 4.24 lb
- 1 lb liquid propane = 0.236 gallon = 21,548 BTU = ~6.3 kWh
- 1 liter liquid propane = ~24,180 BTU = ~7.1 kWh (international reference)
Vapor propane (after the regulator)
- 1 cubic ft propane vapor = ~2,516 BTU
- 1 gallon liquid propane = ~36.4 cubic ft vapor at standard pressure
- 1 lb liquid propane = ~8.6 cubic ft vapor
Common cylinder sizes
- 20 lb cylinder = 4.7 gallons usable (at 80% fill)
- 30 lb cylinder = ~7 gallons usable
- 40 lb cylinder = ~9 gallons usable
- 100 lb cylinder = ~23 gallons usable
Common ASME tank sizes (usable at 80% fill)
- 120 gallon = ~96 gallons usable
- 250 gallon = ~200 gallons usable
- 500 gallon = ~400 gallons usable
- 1,000 gallon = ~800 gallons usable
Cost-per-BTU comparisons
To compare propane against other fuels on a cost-per-million-BTU basis:
- Propane at $2.80/gallon = $30.60 per million BTU
- Heating oil at $4.00/gallon = $28.90 per million BTU (138,500 BTU/gal)
- Natural gas at $1.20/therm = $12 per million BTU (100,000 BTU/therm)
- Electricity at $0.15/kWh = $43.95 per million BTU (resistance heating)
FAQ
Why does propane weigh different per gallon at different temperatures?
Liquid propane expands and contracts with temperature — about 0.1% per °F. The 4.24 lb/gallon figure is referenced at 60°F. At 100°F, a gallon weighs slightly less (volume larger); at 0°F, slightly more. The 80% fill rule exists specifically to accommodate this thermal expansion.
How do I convert kWh to gallons?
Divide kWh by 26.8 (the kWh-equivalent of one gallon at 100% conversion). For real-world appliance efficiency, multiply by the appliance AFUE — e.g. for a 95% AFUE furnace, divide kWh by (26.8 × 0.95) = 25.5.