The Online Encyclopedia and Dictionary






British thermal unit

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.
59F ≡ 1054.804 Chiefly American. Uses the 15 C calorie, itself defined as exactly 4185.5 J (Comit international 1950; PV, 1950, 22, 79-80)
60F ≈ 1054.68 Chiefly Canadian
63F ≈ 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:

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 BTU59F whilst the E.C. uses the BTUIT.

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

Last updated: 05-13-2005 07:56:04