`E = 1,027 (BTUs)/ (ft^3)`

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The Energy in Natural Gas is 1,027 BTUs per cubic foot, which is equal to approximately 137.3 BTUs / gallon. 

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BTUs and Common Energy Sources

BTU stands for British Thermal Unit, and it is the amount of energy needed to heat one pound of water by one degree Fahrenheit.  

Many common energy sources are used to generate energy measured in BTUs.  The following is an excerpt from a U.S. Department of Energy guide listing the BTUs generated by energy products:

BTU Content of Common Energy Sources

• Energy in Crude Oil: 1 barrel (42 gallons) of crude oil = 5,800,000 BTU
• Energy in Propane: 1 gallon of propane = 91,033 BTU
• Energy in Natural Gas: 1 cubic foot of natural gas = 1,027 BTU
• Energy in Heating Oil: 1 gallon of heating oil = 138,690 BTU
• Energy in Diesel: 1 gallon of diesel fuel = 138,690 BTU
• Energy in Gasoline:  1 gallon of gasoline = 123,238 BTU
• Energy in Coal: 1 short ton of coal = 19,977,000 BTU
• Energy in Electricity: 1 kilowatt-hour of electricity = 3,412 BTU