Definition

The therm (symbol thm) is a non-SI unit of heat energy used to measure the consumption of natural gas. In the U.S., the legal definition (made in 1968) is that the therm equals 105.4804 megajoules. The European Union's definition, made in 1979 using the more current IT Btu, is 105.5060 megajoules.

Gas is metered by volume, often in units of hundreds of cubic feet (CCF), but, since the heating value of natural gas varies with the purity of the product, it is sold by its heating value or heat content. Volume is measured in cubic feet and heat value in Btu's.

To standardize the measurement of natural gas, certain adjustments are made on a typical natural gas bill. The heat content of a cubic foot of gas may contain between 1,000 and 1,060 Btu's. A standard cubic foot is 1,031 Btu's. For purposes of measurement for sale, the actual Btu content is determined and a factor applied to convert to standard cubic feet or to the heat content of standard cubic feet.

History

The names of the unit comes from the Greek word for heat, therme.

Conversions

Further reading

• How Many? A Dictionary of Units of Measurement, Russ Rowlett, Center for Mathematics and Science Education, University of North Carolina at Chapel Hill
• SI base units and SI derived units, the Physics Laboratory, National Institute of Standards and Technology
• Energy conversion tool at unitconversion.org