The British thermal unit (BTU) is a non-metric unit of energy, used in the United States and, to a certain extent, the UK. The SI unit is the joule (J), which is used by most other countries. A BTU is defined as the amount of heat required to raise the temperature of one pound avoirdupois of water by one degree Fahrenheit; specifying the temperature range over which this happens leads to a number of slightly different BTU values, varying over a range of about 0.5%:

Name	Value (J)	Notes
39°F	≈ 1059.67	Uses the calorie value of water at its maximum density (39.1–39.2°F)
Mean	≈ 1055.87	Uses a calorie averaged over the 32°F to 212°F range
IT	≡ 1055.05585262	The most widespread BTU, uses the International [Steam] Table (IT) calorie, itself defined for water at 14.5°C (58.1°F) by the Fifth International Conference on the Properties of Steam , held in London in July 1956.
59°F	≡ 1054.804	Chiefly American. Uses the 15°C calorie, itself defined as exactly 4185.5 J (Comité international 1950; PV, 1950, 22, 79-80)
60°F	≈ 1054.68	Chiefly Canadian
63°F	≈ 1054.6	Possibly apocryphal
ISO	≡ 1054.5	ISO 31 Quantities and units (?)
Thermochemical	≡ 1054.35026444	(Calculated from 9489.152 380 4 ÷ 9) Uses the thermochemical calorie of exactly 4184 J

The BTU is often used to describe the heat value of fuels, or the heating and cooling capacity of a system (such as a barbecue grill).

One BTU is approximately:

• 252–253 cal (calories)
• 778–782 ft lbf (feet-pounds-force)
• 1054-1060 joules

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

• 1 horsepower is approximately 2500 BTU/hour
• 1 Watt is approximately 3.4 BTU/hour
• 1000 BTU/hour is approximately 290 W

A unit called the quad (short for quadrillion) is defined as 10¹⁵ BTUs, which is about 1.055×10¹⁸ joules, and the 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 E.C. uses the BTU_IT.

The BTU should not be confused with the Board of Trade Unit (B.O.T.U.), which is a much larger quantity of energy.

See also

• conversion of units
• metrication

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