The therm (symbol, thm) is a non-SI unit of heat energy equal to 100,000 British thermal units (BTU), and approximately 105 megajoules, 29 kilowatt-hours, 25200 kilocalories and 25.2 thermies. One therm is the energy content of approximately 100 cubic feet (2.83 cubic metres) of natural gas at standard temperature and pressure. However, the BTU is not standardised worldwide, with slightly different values in the EU, UK, and USA, meaning that the energy content of the therm also varies by territory.

Natural gas meters measure volume and not energy content, and given that the energy density varies with the mix of hydrocarbons in the natural gas, a 'therm factor' is used by natural gas companies to convert the volume of gas used to its heat equivalent, usually being expressed in units of 'therms per CCF' (CCF is an abbreviation for 100 cubic feet). Higher than average concentration of ethane, propane or butane will increase the therm factor and the inclusion of non-flammable impurities, such as carbon dioxide or nitrogen will reduce it. The Wobbe Index of a fuel gas is also sometimes used to quantify the amount of heat per unit volume burnt.