The amount of energy that is obtained from the burning of a unit volume of natural gas is measured in British thermal units (Btu). One Btu is the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit at normal atmospheric pressure (14.7 pounds per square inch). A cubic foot of natural gas on the average gives off 1,000 Btu, but the range of values is 500 to 1,500 Btu.

Energy content of natural gas varies because natural gas accumulations vary in the amount and types of energy gases they contain: the more non-combustible gases in a natural gas, the lower the Btu value. In addition, how much of any energy gas that is present in a natural gas accumulation - the mix of combustible gases - also influences the Btu value of natural gas. The more carbon atoms in a hydrocarbon gas, the higher its Btu value. To illustrate: methane typically represents more than 80 percent of energy gases. Methane contains one carbon atom per molecule; burning one cubic foot of methane gives off 1,012 Btu. Butane, possessing four carbon atoms, has a Btu value more than three times larger than that of methane. Molecular hydrogen, on the other hand, though combustible, contains no carbon atoms; its Btu value is three times smaller than that of methane.