**Convert**
from Btu_{IT} into other major units of energy.

**Convert**
from Btu_{IT} per hour into other major units of power.

A unit of energy, 1876 – present, usually referred to as a Btu (pronounced “bee tee u”). Symbol, Btu, but see below. In the 19ᵗʰ century, sometimes called a calory.¹

Originally defined as the quantity of heat needed to raise the temperature of 1 pound avoirdupois of air-free water 1°F under a constant pressure of 1 atmosphere, starting at the temperature at which water is most dense, 39.1°F. This is about the amount of energy released when the tip of a kitchen match burns.

Since the calorie, another measure of a quantity of heat, is also defined in terms of a temperature interval and the mass of water heated, any definition of a calorie implies a definition of a Btu.

Name of unit | Symbol | Equivalent in joules |
---|---|---|

International Table Btu. Based on the
definition of the International Table calorie (exactly 4.1868 J) at the
Fifth International Conference on the Properties of Steam (London,
July 1956). |
Btu |
exactly 1055.055 852 62 joules |

thermochemical Btu. Based on the
definition of the thermochemical calorie (exactly 4.1840 joules) by the U.S.
Bureau of Standards in 1953. |
Btu |
approximately 1054.350 joules |

mean Btu. ¹⁄_{180} of the quantity of heat
needed to raise the pound of water from 32° F to 212° F |
1055.87 joules | |

39°F Btu | 1059.67 joules | |

59°F Btu | 1054.80 joules | |

60°F Btu | 1054.68 joules |

Formerly abbreviated BTU, B. T. U., and in Britain, B. Th. U. The abbreviation B.T.U. was also used to identify units approved by the London Board of Trade, such as the B.T.U. ohm or B. T. unit of output.

1. Latimer Clark.

*A Dictionary of Metric and Other Useful Measures.*

London: E & F.N. Spon, 1891.

Page 24.

R. W. Powell.

History of the British Thermal Unit.

*Nature*, vol 149, no. 3784, 9 May 1942, page 525-526.

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Last revised: 1 August 2011.