The British thermal unit (BTU or Btu) is a unit of heat; it is 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. Heat is now known to be equivalent to energy. The modern SI unit for heat and energy is the joule (J); one BTU equals about 1,055 J (varying within the range 1,054–1,060 J depending on the specific definition; see below).
Another legacy unit for energy in the metric system is the calorie, which is defined as the amount of heat required to raise the temperature of one gram of water by one degree Celsius. While units of heat are often supplanted by energy units 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 million BTUs.