British thermal unit

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_IT	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_th	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

Other abbreviations

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.