Code of Federal Regulations Title 18. Conservation of Power and Water Resources § 18.284.4 Reporting

(a) Reports in MMBtu. All reports filed pursuant to this part must indicate quantities of natural gas in MMBtu's. An MMBtu means a million British thermal units. A British thermal unit or Btu means the quantity of heat required to raise the temperature of one pound avoirdupois of pure water from 58.5 degrees to 59.5 degrees Fahrenheit, determined in accordance with paragraphs (b) and (c) of this section.

(b) Measurement. The Btu content of one cubic foot of natural gas under the standard conditions specified in paragraph (c) of this section is the number of Btu's produced by the complete combustion of such cubic foot of gas, at constant pressure with air of the same temperature and pressure as the gas, when the products of combustion are cooled to the initial temperature of the gas and air and when the water formed by such combustion is condensed to a liquid state.

(c) Standard conditions. The standard conditions for purposes of paragraph (b) of this section are as follows:  The gas is saturated with water vapor at 60 degrees Fahrenheit under a pressure equivalent to that of 30.00 inches of mercury at 32 degrees Fahrenheit, under standard gravitational force (980.665 centimeters per second squared).