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The British thermal unit (BTU or Btu) is a unit of energy used in the United States, particularly in the power, steam generation, and 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countries (such as Canada, the United Kingdom, and sometimes in New Zealand), it is increasingly an outmoded and outdated unit of measure. Elsewhere (and always in scientific use) the BTU has been replaced by the SI unit of energy, the joule (J).

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 understood, though this is often confusingly abbreviated to just "BTU". In the United Kingdom and other parts of the world it is written BTU.

The unit MBTU was defined as one thousand BTU presumably from the Roman numeral system where "M" stands for one thousand (1,000). There is currently a social push^{[citation needed]} to redefine MBTU as one million (1,000,000) BTU, since the metric system uses "M" to mean mega, or 10⁶. To avoid confusion many companies and engineers use MMBTU to represent one million (1,000,000) 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 water by one degree Fahrenheit. Melting a pound of ice at 32 °F requires 143 BTU. 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%:

One BTU is approximately:

• 1,054—1,060 joules
• 252—253 cal (calories, small)
• 0.252—0.253 kcal (kilocalories)
• 25,031—25,160 ft·pdl (foot-poundal)
• 778—782 ft·lbf (foot-pounds-force)

Other conversions:

• In natural gas, by convention 1 MMBtu (1 million BTU, sometimes written "mmBTU") = 1.054615 GJ. Conversely, 1 gigajoule is equivalent to 26.8 m³ of natural gas at defined temperature and pressure. So, 1 MMBtu = 28.263682 m³ of natural gas at defined temperature and pressure.
• 1 standard cubic foot of natural gas ≈ 1030 BTU (between 1010 BTU and 1070 BTU, depending on quality)

The BTU per hour (BTU/h) is the unit of power most commonly associated with the BTU.

• 1 watt is approximately 3.41 BTU/h
• 1000 BTU/h is approximately 293 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. It is the amount of power needed to melt one short ton of ice in 24 hours, and is approximately 3.51 kW.
• 1 therm is defined in the United States and European Union as 100,000 BTU — but the U.S. uses the BTU_{59 °F} whilst the EU uses the BTU_IT.
• 1 quad (energy) (short for quadrillion BTU) is defined as 10¹⁵ BTU, which is about one 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/year in 2005. [1]. 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 about 3412 BTU).

• Conversion of units
• Metrication

• The Units of Measurement Regulations 1995
• Natural Gas: A Primer