The British thermal unit (BTU or Btu) is a unit of energy used in the power, steam generation, heating and air conditioning industries. Although it is still used 'unofficially' in metric English-speaking countries (such as Canada, the United Kingdom, and sometimes in New Zealand), its use has declined or has been replaced in other parts of the world. In scientific contexts the BTU has largely been replaced by the SI unit of energy, the joule (J), though it may be used as a measure of agricultural energy production (BTU/kg).
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, that is, BTU divided by hour) is understood, though this is often confusingly abbreviated to just "BTU".
The unit MBTU was defined as one thousand BTU presumably from the Roman numeral system where "M" stands for one thousand (1,000). This is easily confused with the SI mega (M) prefix, which adds a factor of one million (1,000,000). To avoid confusion many companies and engineers use MMBTU to represent one million BTU. Alternatively a therm is used representing 100,000 or 105 BTU, and a quad as 1015 BTU.
A BTU is defined as the amount of heat required to raise the temperature of one pound of liquid water by one degree from 60° to 61°Fahrenheit at a constant pressure of one atmosphere. As is the case with the calorie, several different definitions of the BTU exist, which are based on different water temperatures and therefore vary by up to 0.5%:
|Name or temperature||Value (J)||Notes|
|39 °F||≈ 1059.67||Uses the calorie value of water at its maximum density (4 °C)|
|Mean||≈ 1055.87||Uses a calorie averaged over water temperatures 0 °C to 100 °C|
|IT||≡ 1055.05585262||The most widespread BTU, uses the International [Steam] Table (IT) calorie, which was defined by the Fifth International Conference on the Properties of Steam (London, July 1956) to be exactly 4.1868 J|
|ISO||≡ 1055.056||International standard ISO 31-4 on Quantities and units—Part 4: Heat, Appendix A. This value uses the IT calorie and is rounded to a realistic accuracy |-||59 °F||≡ 1054.804||Chiefly American. Uses the 15 °C calorie, itself now defined as exactly 4.1855 J (Comité international 1950; PV, 1950, 22, 79–80)|
|60 °F||≈ 1054.68||Chiefly Canadian|
|63 °F||≈ 1054.6|
|Thermochemical||≡ 1054.35026444||Uses the "thermochemical calorie" of exactly 4.184 J|
One BTU is approximately:
The BTU per hour (BTU/h) is the unit of power most commonly associated with the BTU. The term is sometimes shortened to BTU hour (BTU.h) but both have the same meaning.
The BTU should not be confused with the Board of Trade Unit (B.O.T.U.), which is a much larger quantity of energy (1 kW·h, or about 3412 BTU).