therms

Therm is a non-SI unit of energy used primarily in the natural gas industry in the United States and Canada to express the energy content of gas. One therm is defined as 100,000 British thermal units (BTU), which is approximately 105.5 megajoules or about 29.3 kilowatt-hours. Because the therm measures energy rather than volume, the energy contained in a given amount of gas depends on its composition and heating value, so therms are used for pricing and settlement rather than to describe gas volume.

A related unit is the decatherm (Dth), equal to 10 therms or 1,000,000 BTU (about 1,055 megajoules or 293 kilowatt-hours). In wholesale markets and many billing contexts, energy is quoted in therms or decatherms rather than in cubic feet or cubic meters.

Usage and context: The therm is commonly applied to transactions, contracts, and invoices for natural gas. It provides a consistent energy basis across varying gas compositions, complementing volume-based measures. When converting from gas volume (for example, cubic feet) to therms, the heating value of the gas—often expressed in BTU per cubic foot—must be used, since the energy content per unit volume can vary.

Not a standard SI unit, the therm remains a legacy unit in North American energy trading. It is part of the broader family of energy units alongside the BTU and the decatherm.

See also: British thermal unit, BTU; decatherm, Dth; natural gas; heating value; energy units.