The British thermal unit (Btu or BTU) is a unit of heat; it is defined as the amount of heat required to raise the temperature of one pound of water by one Fahrenheit degree. It is also part of the United States customary units. Its counterpart in the old metric system is the calorie, which is defined as the amount of heat required to raise the temperature of one gram of water by one Celsius degree. Heat is now known to be equivalent to energy, for which the SI unit is the joule; one BTU is about 1055 joules. While units of heat are often supplanted by energy units in scientific work, they are still used in many fields. For example, in the United States the price of natural gas is quoted in dollars per million BTUs.
A BTU was originally defined as the amount of heat required to raise the temperature of 1 avoirdupois pound of liquid water by 1 Fahrenheit degree at a constant pressure of one atmosphere. There are several different definitions of the BTU that are now known to differ slightly. This reflects the fact that the temperature change of a mass of water due to the addition of a specific amount of heat (calculated in energy units, usually joules) depends slightly upon the water's initial temperature. As seen in the table below, definitions of the BTU based on different water temperatures vary by up to 0.5%.
|Thermochemical||≈ 1054.3503||Originally, the thermochemical BTU was defined as the heat required to raise the temperature of one pound of water from its freezing point to its boiling point, divided by the temperature difference (180 °F). The similar, thermochemical calorie was defined as the heat required to raise the temperature of one gram of water from freezing to boiling divided by the temperature difference in Celsius (100 °C). The International Standards Organization now redefines the thermochemical calorie as exactly 4.184 J. The thermochemical BTU is then defined using the conversions from grams to pounds and from Celsius to Fahrenheit.|
|59 °F (15.0 °C)||≈ 1054.80||Used for U.S. natural gas pricing.|
|60 °F (15.6 °C)||≈ 1054.68||Chiefly Canadian.|
|39 °F (3.9 °C)||≈ 1059.67||Uses the calorie value of water at its maximum density (4 °C, 39.2 °F).|
|IT||= 1055.05585||An early effort to define heat units directly in terms of energy units, and hence to remove the direct association with the properties of water, was taken by the International Steam Table Conferences. These conferences originally adopted the simplified definition that 860 "IT" calories corresponded to exactly 1 international watt-hour (not the same as a modern watt-hour). This definition ultimately became the statement that 1 IT calorie is exactly 4.1868 J. The BTU is then calculated from the calorie as is done for the thermochemical definitions of the BTU and the calorie.|
|ISO||≡ 1055.06||International standard ISO 31-4 on Quantities and units—Part 4: Heat. This value uses the IT calorie and is rounded to a realistic precision.|
Units kBtu are used in building energy use tracking and heating system sizing. Energy Use Index (EUI) represents kBtu per square foot of conditioned floor area. "k" stands for 1,000.
The units MBtu and MMBtu are used in the natural gas and other industries to indicate 1,000 and 1,000,000 BTUs, respectively. However, there is an ambiguity in that the metric system uses the prefix "M" to indicate one million (1,000,000), and consequently "MBtu" is often used to indicate one million BTUs. Some authors have deprecated the use of MBtu, and avoided its use to reduce confusion. 
Energy analysts accustomed to the metric "k" for 1,000 are more likely to use MBtu to represent one million, especially in documents where M represents one million in other energy or cost units, such as MW, MWh and $.
One Btu is approximately:
- 1.054 to 1.060 kJ (kilojoules)
- 0.2931 W⋅h (watt hours)
- 252 to 253 cal (calories)
- 0.25 kcal (kilocalories)
- 25,031 to 25,160 ft⋅pdl (foot-poundal)
- 778 to 782 ft⋅lbf (foot-pounds-force)
- 5.40395 (lbf/in2)⋅ft3
- 1.730735 W/(m⋅K)
A Btu can be approximated as the heat produced by burning a single wooden kitchen match or as the amount of energy it takes to lift a one-pound (0.45 kg) weight 778 feet (237 m).
For natural gas
- In natural gas pricing, the Canadian definition is that 1000000 Btu ≡ 1.054615 GJ.
- 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 energy to volume. 1 cubic foot (28 litres) of average natural gas yields ≈ 1030 Btu (between 1010 Btu and 1070 Btu, depending on quality, when burned)
- As a coarse approximation, 1,000 cubic feet (28 m3) of natural gas yields ≈ 1000000 Btu ≈ 1 GJ.
- For natural gas price conversion 1000 m3 ≈ 36.9 million Btu and 1000000 Btu ≈ 27.1 m3
As a unit of power
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". MBH—thousands of Btus per hour—is also common.
- 1 watt is approximately 3.412142 Btu/h
- 1000 Btu/h is approximately 293.1 W
- 1 horsepower is approximately 2,544 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 rate of heat transfer needed to freeze 1 short ton (907 kg) of water into ice in 24 hours.
- In the United States and Canada, the R-value that describes the performance of thermal insulation is typically quoted in square foot degree Fahrenheit hours per British thermal unit (ft2⋅°F⋅h/Btu). For one square foot of the insulation, one BTU per hour of heat flows across the insulator for each degree of temperature difference across it.
- 1 therm is defined in the United States and European Union as 100,000 Btu—but the U.S. uses the Btu59 °F while the EU uses the BtuIT. United Kingdom regulations were amended to replace therms with joules with effect from January 1, 2000. As of 2013[update] the therm is still used in natural gas pricing in the United Kingdom.
- 1 quad (short for quadrillion Btu) is 1015 Btu, which is about 1 exajoule (1.055×1018 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. One quad/year is about 33.43 gigawatts.
The Btu should not be confused with the Board of Trade Unit (B.O.T.U.), an obsolete UK synonym for kilowatt hour (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/kWh (3.1 kWh/kWh), an efficiency of 32–33%.
The centigrade heat unit (CHU) is the amount of heat required to raise the temperature of one pound of water by one Celsius degree. It is equal to 1.8 BTU or 1899 joules. This unit was sometimes used in the United Kingdom as an alternative to BTU but is now obsolete.
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- "Henry Hub Natural Gas Spot Price". U.S. Energy Information Administration. Archived from the original on 1 August 2017.
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- The pound is now defined as 453.59237 grams; see "Appendix C of NIST Handbook 44, Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices, General Tables of Units of Measurement" (PDF). United States National Bureau of Standards. p. C-12. Archived from the original (PDF) on 26 November 2006. One degree Fahrenheit is now defined as exactly 5/9 of a degree Celsius.
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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.
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