The therm (symbol, thm) is a non-SI unit of heat energy equal to 100000 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).
One therm is equal to about 105.5 megajoules, 25200 kilocalories, or 29.3 kilowatt-hours. One therm can also be provided by about 96.7 cubic feet (2.74 m3) of natural gas. The therm sometimes has been confused with the thermie. The names of both units come from the Greek word for heat.
- Therm (EC) ≡ 100000 BTUISO
- Therm (US) ≡ 100000 BTU59 °F
- = 105480400 joules
- ≈ 29.3001111111111 kWh.
- Therm (UK) ≡ 105505585.257348 joules
- ≡ 29.3071070159300 kWh
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.
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.
- IEEE Std 260.1-2004
- Official Journal L 073, P. 0114 27 March 1972
- 15 USC Chapter 6 Archived 2006-01-25 at the Wayback Machine
- The Units of Measurement Regulations 1995
- Jerry Knight (22 January 1978). "Gas Utilities Stepping Up Efforts to Add Customers". The Washington Post. Retrieved 1 June 2016.
- epa.gov "Greenhouse Gases Equivalencies Calculator - Calculations and References