High Quality Content by WIKIPEDIA articles! The British thermal unit (symbol BTU or Btu) is a traditional unit of energy equal to about 1055 joules. It is approximately the amount of energy needed to heat 1 pound (0.454 kg) of water, which is exactly one tenth of a UK gallon or about 0.1198 US gallons, from 39°F to 40°F (3.8°C to 4.4°C). The unit is most often used in the power, steam generation, heating and air conditioning industries. In scientific contexts the BTU has largely been replaced by the SI unit of energy, the joule, though it may be used as a measure of agricultural energy production (BTU/kg). It is still used unofficially in metric English-speaking countries (such as Canada), and remains the standard unit of classification for air conditioning units manufactured and sold in many non-English-speaking metric countries.