Quantities of natural gas are measured in cubic metres (at a pressure of 75,000 Pascal and a temperature of 15º C) or in cubic feet (at the same pressure and temperature). Normally, gas production from wells and supplies to power plants are measured in thousands or millions of cubic feet (Mcf and MMcf); resources and reserves are calculated in trillions of cubic feet (Tcf).

The amount of energy that is obtained from the burning of a volume of natural gas is measured in British thermal units (Btu). The value of natural gas is calculated by its Btu content. One Btu is the quantity of heat required to raise the temperature of one pound of water of 1 degree Fahrenheit at atmospheric pressure. A cubic foot of natural gas on the average gives off 1,000 Btu, but the range of values is between 500 and 1,500 Btu.

Energy content of natural gas is variable and depends on its accumulations which are influenced by 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, the volumic mass of energy gases which are present in a natural gas accumulation also influences the Btu value of natural gas. The more carbon atoms in a hydrocarbon gas, the higher its Btu value.

Btu analyses of natural gas are done at each stage of the supply chain. Gas chromatographic process analysers are used in order to conduct fractional analysis of the natural gas streams, separating natural gas into identifiable components. The components and their concentrations are converted into a gross heating value in Btu-cubic foot.

The composition of natural gas varies depending on the field, formation or reservoir from which it is extracted. The different hydrocarbons that form natural gas can be separated using their different physical properties as weight, boiling point or vapour pressure. Depending on its content of heavy components, natural gas can be considered as rich (five or six gallons or more of recoverable hydrocarbons per cubic feet) or lean (less than one gallon of recoverable hydrocarbons per cubic feet).

Normally, natural gas as it is when extracted is not suitable for pipeline transportation or commercial use before being processed. Natural gas for commercial distribution is composed almost entirely of methane and ethane, while moisture and other components have been removed. Pipelines set their specifications for the quality of natural gas. In any case, natural gas must be processed in order to remove unwanted water vapour, solids or other contaminants and to get those hydrocarbons that have a higher value as separate products.