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Units of Heat - BTU, Calorie and Joule

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The most common units for heat are

  • BTU (Btu) - British Thermal Unit - also known as a "heat unit" in United States
  • Calorie
  • Joule

BTU - British Thermal Unit

The unit of heat in the imperial system - the BTU - is

  • the amount of heat required to raise the temperature of one pound of water through 1oF (58.5oF - 59.5oF) at sea level (30 inches of mercury).
  • 1 Btu (British thermal unit) = 1055.06 J = 107.6 kpm = 2.931 10-4 kWh = 0.252 kcal = 778.16 ft.lbf = 1.0551010 ergs = 252 cal = 0.293 watt-hours

An item using one kilowatt-hour of electricity generates 3412 Btu.

  • one hundred thousand (105) Btu are called a therm


A calorie is commonly defined as

  • the amount of heat required to raise the temperature of one gram of water 1 oC
  • the kilogram calorie, large calorie, food calorie, Calorie (capital C) or just calorie (lowercase c) is the amount of energy required to raise the temperature of one kilogram of water by one degree Celsius
  • 1 calorie (cal) = 1/860 international watthour (Wh)
  • 1 kcal = 4186.8 J = 426.9 kp m = 1.163 10-3 kWh = 3.088 ft lbf = 3.9683 Btu = 1000 cal

Be aware that alternative definitions exists - in short: 

  • Thermochemical calorie  
  • 4 °C calorie
  • 15 °C calorie
  • 20 °C calorie
  • Mean calorie
  • International Steam Table calorie (1929)
  • International Steam Table calorie (1956)
  • IUNS calorie (Committee on Nomenclature of the International Union of Nutritional Sciences)

The calorie is outdated and commonly replaced by the SI-unit Joule.


The unit of heat in the SI-system the Joule is

  • a unit of energy equal to the work done when a force of one newton acts through a distance of one meter
  • 4.184 joule of heat energy (or one calorie) is required to raise the temperature of a unit weight (1 g) of water from 0oC to 1oC, or from 32oF to 33.8oF
  • 1 J (Joule) = 0.1020 kpm = 2.778 10-7 kWh = 2.389 10-4 kcal = 0.7376 ft.lbf = 1 kg.m2/s2 = 1 watt second = 1 Nm = 9.478 10-4 Btu
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