The number of British thermal units (BTU, btu) produced by the combustion at constant pressure of one cubic foot of a fuel, under standard conditions of temperature and pressure.

A British thermal unit is a widely used unit of measure, generically defined as the average amount of energy required to produce a change in temperature of one degree Fahrenheit (F) in one pound of pure liquid water; often specified as occurring at standard atmospheric pressure and a specified temperature increase; equivalent to about 1055 joules or 252 calories.