The therm (symbol thm)'s a non-SI unit of heat energy equal to 100,000 British thermal units (BTU). It's approximately the energy equivalent of burning 100 cubic feet (often referred to as 1 Ccf) of natural gas. A therm factor's 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's usually in the units therms/Ccf. It'll vary with the mix of hydrocarbons in the natural gas. Natural gas with a higher than averag… (More on Therm)