The British thermal unit (BTU or Btu) is a non-metric unit of energy, used in the United States and, to a lesser extent, the UK (where it is generally only used for heating systems). 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 4.184 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, small)
• 0.252–0.253 food calorie
• 778–782 ft·lbf (foot-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 2540 BTU/hour
• 1 watt is approximately 3.4 BTU/hour
• 1000 BTU/hour is approximately 293 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