The British thermal unit (Btu or BTU) is a traditional unit of heat; it is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. It is also part of the United States customary units. Its counterpart in the 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 degree Celsius. 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. As examples, 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 degree Fahrenheit 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%.
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.
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.
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. There is an ambiguity in that the metric system uses the prefix "M" to indicate one million (1,000,000), and "MBtu" is also used to indicate one million BTUs. Because of this ambiguity, some authors have deprecated the use of MBtu.
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 $.
The unit therm is used to represent 100,000 (or 105) BTUs. A decatherm is 10 therms or one MBtu (million Btu). The unit quad is commonly used to represent one quadrillion (1015) BTUs.
In natural gas pricing, the Canadian definition is that 1 MMBtu ≡ 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 the number of Btu to volume. 1 standard cubic foot of average natural gas yields ≈ 1030 Btu (between 1010 Btu and 1070 Btu, depending on quality, when burned)
As a coarse approximation, 1000 ft3 of natural gas yields ≈ 1 MMBtu ≈ 1 GJ
For natural gas price conversion 1000 m3 ≈ 36.906 MMBtu and 1 MMBtu ≈ 27.096 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 horsepower is approximately 2544 Btu/h, or Btu/s
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. The therm is 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.), 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 10500 Btu/kW⋅h, 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 degree Celsius. 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.
^In a short note, Woledge notes that the actual technical term "British thermal unit" apparently originated in the United States, and was subsequently adopted in Great Britain. See Woledge, G. (30 May 1942). "History of the British Thermal Unit". Nature. 149 (149): 613. Bibcode:1942Natur.149..613W. doi:10.1038/149613c0.
^ abThe Btu used in US natural gas pricing is "the amount of heat required to raise the temperature of one (1) pound of avoirdupois pure water from fifty-eight and five tenths degrees (58.5) Fahrenheit to fifty-nine and five tenths degrees (59.5) Fahrenheit at a constant pressure of 14.73 pounds per square inch absolute." See "Chapter 220: Henry Hub Natural Gas Futures"(PDF). NYMex Rulebook. New York Mercantile Exchange (NYMex). Archived(PDF) from the original on 10 November 2016. Retrieved 6 January 2017.