British thermal unit

The British thermal unit (Btu) is a unit of measure of heat, which is a form of energy. It was originally defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. It is also part of the United States customary units. In terms of joules (J), the SI unit for energy, one Btu equals about 1,055 J (varying within the range of 1,054–1,060 J depending on the specific definition of Btu; see below).

While units of heat are often supplanted by units of energy in scientific work, they are still used in some fields. For example, in the United States the price of natural gas is quoted in dollars per the amount of natural gas that would give 1 million Btu (1 "MMBtu") of heat energy if burned.