British thermal unit

"BTU" redirects here. For other uses, see BTU (disambiguation).

The British thermal unit (BTU or Btu) is a traditional unit of energy equal to about 1,055.05585 joules. It is approximately the amount of energy needed to heat 1 pound (0.454 kg) of water from 39 °F (3.9 °C) to 40 °F (4.4 °C). The unit is most often used in the power, steam generation, heating and air conditioning industries. In scientific contexts the BTU has largely been replaced by the SI unit of energy, the joule, though it may be used as a measure of agricultural energy production (BTU/kg). It is still used unofficially 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.

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 the correct unit, though this is often 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 multiplies by 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.

Definitions

A BTU is defined as amount of heat required to raise the temperature of one 1 pound (0.454 kg) of liquid water by 1 °F (0.56 °C) 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%: A BTU can be approximated as the heat produced by burning a single wooden match or as the amount of energy it would take to lift a one-pound weight to a height of 778 feet (237 m).

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, 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	≡ 1054.35026444	Uses the "thermochemical calorie" of exactly 4.184 J

Conversions

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)

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 yields ≈ 1030 BTU (between 1010 BTU and 1070 BTU, depending on quality, when burned)

Associated units

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.

• 1 watt is approximately 3.41214 BTU/h
• 1000 BTU/h is approximately 293.071 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. 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).

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 kWh of electrical energy. A typical coal-fired power plant works at 10,500 BTU/kWh, an efficiency of 32-33%.

See also

• Conversion of units
• Latent heat
• Metrication

External links

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