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The **British thermal unit** (**BTU** or **Btu**) is a traditional unit of energy equal to about 1055 joules. It is the amount of energy needed to cool or heat one pound of water by one degree Fahrenheit. In science, the joule, the SI unit of energy, has largely replaced the BTU.

The BTU is most often used as a measure of power (as BTU/h) in the power, steam generation, heating, and air conditioning industries, and also 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.^{[1]} In North America, the heat value (energy content) of fuels is expressed in BTUs.

A BTU is the amount of heat required to raise the temperature of 1 pound (0.454 kg) of liquid water by 1 °F (0.56 °C) at a constant pressure of one atmosphere.^{[2]}^{[3]} As with the calorie, several definitions of the BTU exist, because the temperature response of water to heat energy is non-linear. This means that the change in temperature of a water mass caused by adding a certain amount of heat to it will be a function of the water's initial temperature. Definitions of the BTU based on different water temperatures can therefore vary by up to 0.5%. A BTU can be approximated as the heat produced by burning a single wooden match^{[4]} or as the amount of energy it takes to lift a one-pound weight 778 feet (237 m).^{[5]}

Nominal temperature | BTU equivalent in joules | Notes |
---|---|---|

39 °F (3.9 °C) | ≈ 1059.67 | Uses the calorie value of water at its maximum density (4 °C or 39.2 °F) |

Mean | ≈ 1055.87 | Uses a calorie averaged over water temperatures 0 to 100 °C (32.0 to 212.0 °F) |

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,^{[6]} Appendix A. This value uses the IT calorie and is rounded to a realistic accuracy |

59 °F (15.0 °C) | ≡ 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 (15.6 °C) | ≈ 1054.68 | Chiefly Canadian |

63 °F (17.2 °C) | ≈ 1054.6 | |

Thermochemical ("Th"^{[7]}) | ≡ 1054.35026444 | Uses the "thermochemical calorie" of exactly 4.184 J |

The unit **MBtu** or **mBtu** was defined as one thousand BTU, presumably from the Roman numeral system where "M" or "m" stands for one thousand (1,000). This notation is easily confused with the SI **mega- (M)** prefix, which denotes multiplication by a factor of one million (×10^{6}), or with the SI **milli- (m)** prefix, which denotes division by a factor of one thousand (×10^{−3}). To avoid confusion, many companies and engineers use the notation **MMBtu** or **mmBtu** to represent one million BTU (although, confusingly, MM in Roman numerals would traditionally represent 2,000) and in many contexts this form of notation is deprecated and discouraged in favour of the more modern SI prefixes. Alternatively, the term **therm** may be used to represent 100,000 (or 10^{5}) BTU, and **quad** for 10^{15} BTU. Some companies also use **BtuE6** in order to reduce confusion between 10^{3} BTU and 10^{6} BTU.^{[8]}

One BTU is approximately:

- 1.054 to 1.060 kJ (kilojoules)
- 0.293071 W·h (watt hours)
- 252 to 253 cal (calories, or "little calories")
- 0.25 kcal (kilocalories, "large calories," or "food calories")
- 25,031 to 25,160 ft·pdl (foot-poundal)
- 778 to 782 ft·lbf (foot-pounds-force)
- 5.40395 (lbf/in
^{2})·ft^{3}

- In natural gas, by convention 1 MMBtu (1 million BTU) = 1.054615 GJ.
^{[9]} - The energy content (high or low heating value) of a volume of natural gas varies with the composition of the natural gas, which means there is no universal conversion factor for the number of BTU to volume. 1 standard cubic foot of typical natural gas yields ≈ 1030 BTU (between 1010 BTU and 1070 BTU, depending on quality, when burned)
- As a coarse approximation, 1000 ft
^{3}of natural gas yields ≈ 1 MMBTU ≈ 1 GJ - 1 ft
^{3}of natural gas yields ≈ 1000 BTU - For natural gas price conversion 1000 m
^{3}≈ 35.8 MMBTU and 1 MMBTU ≈ 27.9 m^{3}

When used as a unit of power for heating and cooling systems, BTU *per hour* (BTU/h) is the correct unit, though this is often abbreviated to just "BTU".^{[verification needed]}.

- 1 watt is approximately 3.41214 BTU/h
^{[10]} - 1000 BTU/h is approximately 293.071 W
- 1 horsepower is approximately 2544 BTU/h

- 1
*ton of cooling*, a common unit in North American refrigeration and air conditioning applications, is 12,000 BTU/h (3.52 kW). It is the amount of power needed to freeze 1 short ton (0.893 long ton; 0.907 t) of water into ice in 24 hours. - 1
*therm*is defined in the United States and European Union as 100,000 BTU—but the U.S. uses the BTU_{59 °F}while the EU uses the BTU_{IT}. The therm is used to price natural gas.^{[11]} - 1
*quad*(short for quadrillionBTU) is 10^{15}BTU, which is about one exajoule (1.055×10^{18}J). Quads are used in the United States for representing the annual energy consumption of large economies: for example, the U.S. economy used 99.75 quads in 2005.^{[12]}One quad/year is about 33.43 gigawatts.

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 3,412 BTU).

The BTU is often used to express the conversion-efficiency of heat into electrical energy in power plants. Figures are quoted in terms of the quantity of heat in BTU required to generate 1 kW·h of electrical energy. A typical coal-fired power plant works at 10,500 BTU/kW·h, an efficiency of 32–33%.^{[13]}

**^**"Metrication Problems in the Construction Codes and Standards Sector". Eric.ed.gov. Retrieved 5 April 2014.**^**"What is British thermal unit (Btu)? definition and meaning". Businessdictionary.com. Retrieved 2011-11-11.**^**"BTU Calculator". Retrieved 5 April 2014.**^**Energy and the Environment. Ristinen, Robert A. c. 2006, pg 13**^**Energy and the Environment. Ristinen, Robert A. c. 2006, pg 14**^**International standard ISO 31-4:1992 Quantities and units—Part 4: Heat**^**[1]**^**EIA (2012-09-17). "What are Mcf, BTU, and therms? How do I convert prices in Mcf to BTUs and therms?". US Government.**^**"Energy Measurements". Energy.gov.ab.ca. Retrieved 5 April 2014.**^**2009 ASHRAE Handbook – Fundamentals (I-P Edition). (pp: 38.2). American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc**^**"The GB gas wholesale market". OFGEM. Retrieved Jan 13, 2013. "The wholesale gas market in Britain has one price for gas irrespective of where the gas comes from. This is called the National Balancing Point (NBP) price of gas and is usually quoted in price per therm of gas."**^**Husher, John Durbin.*Crises of the 21st Century: Start Drilling-The Year 2020 Is Coming Fast*, iUniverse, 2009. Page 376.**^**Electric Generation Efficiency, NPC Global Oil & Gas Study, 18 July 2007