Engineering ToolBox - Resources, Tools and Basic Information for Engineering and Design of Technical Applications!

Units of Heat - BTU, Calorie and Joule

The most common units of heat BTU - British Thermal Unit, Calorie and Joule.

Sponsored Links

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The calorie is outdated and commonly replaced by the SI-unit Joule.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

Related Topics


Heating systems - capacity and design of boilers, pipelines, heat exchangers, expansion systems and more.


Work, heat and energy systems.

Related Documents

Sponsored Links

Engineering ToolBox - SketchUp Extension - Online 3D modeling!

3D Engineering ToolBox Extension to SketchUp - add parametric components to your SketchUp model

Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your Sketchup model with the Engineering ToolBox - SketchUp Extension - enabled for use with the amazing, fun and free SketchUp Make and SketchUp Pro . Add the Engineering ToolBox extension to your SketchUp from the SketchUp Pro Sketchup Extension Warehouse!

About the Engineering ToolBox!


We don't collect information from our users. Only emails and answers are saved in our archive. Cookies are only used in the browser to improve user experience.

Some of our calculators and applications let you save application data to your local computer. These applications will - due to browser restrictions - send data between your browser and our server. We don't save this data.

Google use cookies for serving our ads and handling visitor statistics. Please read Google Privacy & Terms for more information about how you can control adserving and the information collected.

AddThis use cookies for handling links to social media. Please read AddThis Privacy for more information.


This page can be cited as

  • The Engineering ToolBox (2004). Units of Heat - BTU, Calorie and Joule. [online] Available at: [Accessed Day Month Year].

Modify access date.

. .


3D Engineering ToolBox - draw and model technical applications! 2D Engineering ToolBox - create and share online diagram drawing templates! Engineering ToolBox Apps - mobile online and offline engineering applications!

6 6

Sponsored Links