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Standard: SI derived unit
Quantity: Energy
Symbol: J
Named after: James Prescott Joule
Expressed in: 1 J =
SI base units 1 kg·m2/s2
CGS units 1×107 erg

The joule (symbol J), named for James Prescott Joule, is the derived unit of energy in the International System of Units. It is the energy exerted by the force of one newton acting to move an object through a distance of one metre. In terms of dimensions:

\rm 1\ J = 1\ N \cdot m = \left ( \frac{kg \cdot m}{s^2} \right ) \cdot m = \frac{kg \cdot m^2}{s^2}=Pa \cdot m^3= 1\ W \cdot s

One joule is defined as the amount of work done by a force of one newton moving an object through a distance of one metre. Other relationships are:




This SI unit is named after James Prescott Joule. As with every SI unit whose name is derived from the proper name of a person, the first letter of its symbol is uppercase (J). When an SI unit is spelled out in English, it should always begin with a lowercase letter (joule), except where any word would be capitalized, such as at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because the "d" is lowercase.
—Based on The International System of Units, section 5.2.

Confusion with newton metre

While it is dimensionally correct to express joules as newton metres or N·m, such use is discouraged[1] by the SI authority to avoid confusion with torque. Torque and energy are fundamentally different physical quantities. For example, adding 1 N·m of torque to 1 N·m of energy gives a dimensionally consistent result of 2 N·m, but this quantity is physically meaningless.

Practical examples

One joule in everyday life is approximately:

  • the energy required to lift a small apple one meter straight up.
  • the energy released when that same apple falls one meter to the ground.
  • the energy released as heat by a person at rest, every hundredth of a second.
  • one hundredth of the energy a person can receive by drinking a drop of beer.
  • the kinetic energy of an adult human moving at a speed of about a handspan every second.
  • the kinetic energy of a tennis ball moving at 23 km/h (14 mph).[2]


For additional examples, see: Orders of magnitude (energy)
SI multiples for joule (J)
Submultiples Multiples
Value Symbol Name Value Symbol Name
10–1 J dJ decijoule 101 J daJ decajoule
10–2 J cJ centijoule 102 J hJ hectojoule
10–3 J mJ millijoule 103 J kJ kilojoule
10–6 J µJ microjoule 106 J MJ megajoule
10–9 J nJ nanojoule 109 J GJ gigajoule
10–12 J pJ picojoule 1012 J TJ terajoule
10–15 J fJ femtojoule 1015 J PJ petajoule
10–18 J aJ attojoule 1018 J EJ exajoule
10–21 J zJ zeptojoule 1021 J ZJ zettajoule
10–24 J yJ yoctojoule 1024 J YJ yottajoule
Common multiples are in bold face


The nanojoule (nJ) is equal to one billionth of one joule. One nanojoule is about 1/160 of the kinetic energy of a flying mosquito.[3]


The microjoule (μJ) is equal to one millionth of one joule. The Large Hadron Collider (LHC) is expected to produce collisions on the order of 1 microjoule (7 TeV) per particle.


The millijoule (mJ) is equal to one thousandth of one joule.


The kilojoule (kJ) is equal to one thousand joules. Food labels in some countries express food energy in kilojoules. One kilojoule is about the amount of solar radiation received by one square metre of the Earth in one second.[4]


The megajoule (MJ) is equal to one million joules, or approximately the kinetic energy of a one-ton vehicle moving at 160 km/h (100 mph).


The gigajoule (GJ) is equal to one billion joules. Six gigajoules is about the amount of chemical energy in a barrel of oil.[5]


The terajoule (TJ) is equal to one trillion joules. About 60 terajoules were released by the bomb that exploded over Hiroshima.[6]


1 joule is equal to:

Units defined exactly in terms of the joule include:

  • 1 thermochemical calorie = 4.184 J[7]
  • 1 International Table calorie = 4.1868 J[7]
  • 1 watt hour = 3600 J
  • 1 kilowatt hour = 3.6×106 J (or 3.6 MJ)
  • 1 ton TNT = 4.184 GJ

See also


  1. ^ From the official SI website: "A derived unit can often be expressed in different ways by combining base units with derived units having special names. Joule, for example, may formally be written newton metre, or kilogram metre squared per second squared. This, however, is an algebraic freedom to be governed by common sense physical considerations; in a given situation some forms may be more helpful than others. In practice, with certain quantities, preference is given to the use of certain special unit names, or combinations of unit names, to facilitate the distinction between different quantities having the same dimension."
  2. ^ Ristinen, Robert A., and Jack J. Kraushaar. Energy and the Environment. 2nd ed. Hoboken, NJ: John Wiley & Sons, Inc., 2006.
  3. ^ CERN - Glossary
  4. ^ "Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to present". Retrieved 2005-10-05. 
  5. ^ IRS publication
  6. ^ Los Alamos National Laboratory report LA-8819, The yields of the Hiroshima and Nagasaki nuclear explosions by John Malik, September 1985. Available online at
  7. ^ a b The adoption of joules as units of energy, FAO/WHO Ad Hoc Committee of Experts on Energy and Protein, 1971. A report on the changeover from calories to joules in nutrition.

External links


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