The British thermal unit (symbol Btu or sometimes BTU) is a traditional unit of energy equal to about 1,055 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 In scientific contexts the Btu has largely been replaced by the SI unit of energy, the joule, though it may be used as a measure of agricultural energy production (Btu/kg).^{[verification needed]} It is still used in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.^{[verification needed]}

In North America, the term "Btu" is used to describe the heat value (energy content) of fuels, and also to describe the power of heating and cooling systems, such as furnaces, stoves, barbecue grills, and air conditioners. When used as a unit of power, Btu per hour (Btu/h) is the correct unit, though this is often abbreviated to just "Btu".^{[verification needed]}