A therm is a measure of the energy content of natural gas and is equal to 100,000 British thermal units (Btu). Many gas utilities bill customers in units of therms. Because gas meters measure volume (typically cubic feet/cf or hundreds of cf/Ccf) and not energy content, a meter reading must be converted to therms using a billing factor that is determined based on the heating value of a natural gas stream. Because heating value varies with the source of the gas, billing factors vary by location. Factors also vary due to meter calibration, delivery pressure, and altitude.
In the above bill, a monthly meter read of 37 Ccf (one hundred cubic feet) was recorded. The billing factor for this meter is 1.085, so the number of therms for which the customer will be billed is calculated: 37 x 1.085 = 40 therms.