The British thermal unit (BTU or Btu) is a measure of heat, which is measured in units of energy. 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. The modern SI unit for energy is the joule (J); one BTU equals about 1,055 J (varying within the range of 1,054–1,060 J depending on the specific definition; see below).

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 the amount of natural gas that would give 1 million BTUs (1 "MMBtu") of heat energy if burned.