Wikipedia
A dekatherm (dth) is a unit of energy used primarily to measure natural gas, developed in about 1972 by the Texas Eastern Transmission Corporation, a natural gas pipeline company. It is equal to 10 therms or 1,000,000 British thermal units (MMBtu). It is also approximately equal to one thousand cubic feet (Mcf) of natural gas or exactly one Mcf of natural gas with a heating value of 1000 Btu/cf.
Natural gas is a mixture of gases containing approximately 80% methane (CH4) and its heating value varies from about 975 Btu/cf (1.026 Mcf/dth) to 1100 Btu/cf (0.91 Mcf/dth) depending on the mix of different gases in the gas stream. Noncombustible carbon dioxide (CO2) lowers the heating value of natural gas. Heavier hydrocarbons such as ethane (C2H6), propane (C3H8), and butane (C4H10) increase its heating value. Since customers who buy natural gas are actually buying heat, gas distribution companies who bill by volume routinely adjust their rates to compensate for this.
To simplify billing, Texas Eastern staff members coined the term decatherm (ten therms) and proposed using calorimeters to measure and bill gas delivered to customers in dekatherms. This would eliminate the constant calculation of rate adjustments to $/Mcf rates in order to assure that all customers received the same amount of heat per dollar. A settlement agreement reflecting the new billing procedure and settlement rates was filed in 1973. The FPC issued an order approving the settlement agreement and the new tariff using dekatherms later that year, Other gas distribution companies also began to use this process.
In spite of the need for adjustments, many companies continue to use cubic feet rather than dekatherms to measure and bill natural gas.