The therm is a non-SI
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...

Units of energy
Because energy is defined via work, the SI unit for energy is the same as the unit of work – the joule , named in honour of James Prescott Joule and his experiments on the mechanical equivalent of heat...

 of heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...

In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

 equal to 100,000 British thermal unit
British thermal unit
The British thermal unit is a traditional unit of energy equal to about 1055 joules. It is approximately the amount of energy needed to heat of water, which is exactly one tenth of a UK gallon or about 0.1198 US gallons, from 39°F to 40°F...

s (BTU). It is approximately the energy equivalent of burning 100 cubic feet
Cubic foot
The cubic foot is an Imperial and US customary unit of volume, used in the United States and the United Kingdom. It is defined as the volume of a cube with sides of one foot in length.-Conversions:- Symbols :...

 (often referred to as 1 Ccf) of natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...


Since meters measure volume and not energy content, a therm factor is used by 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 in the units therms/Ccf. It will vary with the mix of hydrocarbons in the natural gas. Natural gas with a higher than average concentration of ethane
Ethane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane that is an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colorless, odorless gas....

, propane
Propane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central...

 or butane
Butane is a gas with the formula C4H10 that is an alkane with four carbon atoms. The term may refer to any of two structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, butane refers only to the unbranched n-butane isomer; the other one being called "methylpropane" or...

 will have a higher therm factor. Impurities, such as carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 or nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

, lower the therm factor.

The volume of the gas is calculated as if measured at standard temperature and pressure (STP). The heat content of natural gas is solely dependent on the composition of the gas, and is independent of temperature and pressure.

The therm is equal to about 105.5 megajoules, 25,200 kilocalories or 29.3 kilowatt-hours. One therm can also be provided by about 96.7 cubic feet (2.7 m³) of natural gas. The therm sometimes has been confused with the thermie (see below). The names of both units come from the Greek word for heat.

A thermie (th) is a metric unit of heat energy, part of the meter-tonne-second system sometimes used by European engineers. The thermie is equal to the amount of energy required to raise the temperature of 1 tonne of water by 1 °C. The thermie is equivalent to 1,000 kilocalories, 4.1868 megajoules or 3968.3 BTU.


  • Therm (EC) ≡ 100,000 BTUIT
= 105,506,000 joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...

≈ 29.3072222 kWh
The therm (EC) is often used by engineers in the US.
  • Therm (US) ≡ 100,000 BTU59°F
= 105,480,400 joules
≈ 29.3001111 kWh.
  • Therm (UK) = 105,505,585.257 348 joules
≈ 29.30710701583 kWh

Ten therms are a decatherm. Common abbreviations are as follow:
Dth decatherm
MDth thousand decatherms
MMDth million decatherms


United Kingdom regulations were amended to replace therms with joules with effect from 1999. Despite this, natural gas is now usually retailed in kilowatt-hours, although the wholesale UK gas market trades in therms, with a typical forward transaction being for 25,000 therms/day (31 MW). In the United States, however, natural gas is commonly billed in CCFs (100 cubic feet) or therms.

Carbon footprint

According to the Pacific Gas and Electric Company
Pacific Gas and Electric Company
The Pacific Gas and Electric Company , commonly known as PG&E, is the utility that provides natural gas and electricity to most of the northern two-thirds of California, from Bakersfield almost to the Oregon border...

 emissions rate, burning natural gas produces on average 13.446 pounds (6.1 kg) of carbon dioxide per therm.

See also

  • Barrel of oil equivalent
    Barrel of oil equivalent
    The barrel of oil equivalent is a unit of energy based on the approximate energy released by burning one barrel of crude oil. The US Internal Revenue Service defines it as equal to 5.8 × 106 BTU...

  • Conversion of units#Energy, work, or amount of heat
  • Cubic mile of oil
    Cubic mile of oil
    The cubic mile of oil is a unit of energy. It was created by Hew Crane of SRI International to aid in public understanding of global-scale energy consumption and resources....

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