The therm (symbol, thm) is a non-SI unit of heat energy equal to 100,000 British thermal units (BTU). It is approximately the energy equivalent of burning 100 cubic feet (2.83 cubic metres) – often referred to as 1 CCF – of natural gas.
Since natural gas meters measure volume and not energy content, a therm factor is used by natural gas companies to convert the volume of gas used to its heat equivalent, and thus calculate the actual energy use. The therm factor is usually expressed in units of therms per CCF. It will vary with the mix of hydrocarbons in the natural gas. Natural gas with a higher than average concentration of ethane, propane or butane will have a higher therm factor. Impurities, such as carbon dioxide or nitrogen, lower the therm factor.
The volume of the gas is calculated as if measured at standard temperature and pressure (STP).
The therm sometimes has been confused with the thermie. The names of both units come from the Greek word for heat.
10 therms are known as a decatherm (sometimes, dekatherm; commonly abbreviated Dth), which is 1000000 Btu (of whichever type). Further common abbreviations are MDth for a 1000 decatherms, and MMDth for 1000000 decatherms.[failed verification]
United Kingdom regulations were amended to replace therms with joules with effect from 1999, with natural gas usually retailed in the derived unit, kilowatt-hours. Despite this, the wholesale UK gas market trades in therms. In the United States, natural gas is commonly billed in CCFs (hundreds of cubic feet) or therms.