Joule Definition (Unit in Science)

James Joule
James Joule. Henry Roscoe/Wikimedia Commons/Public Domain

The joule (symbol: J) is the basic SI unit of energy. A joule is equal to the kinetic energy of a kilogram mass moving at the speed of one meter per second (one joule is a kg⋅m2⋅s−2). Alternatively, it is the amount of work done on an object when a force of one newton acts in the direction of the object's motion over a distance of one meter (1 joule equal 1 newton meter or N⋅m).

The unit is named for James Prescott Joule. Because it is named for a person, the first letter of the symbol is uppercase (J instead of j). However, when the word is written out, it is written in lowercase (joule instead of Joule, unless it begins a sentence).

Joule Examples

To put the joule into a practical context:

  • One joule is the kinetic energy of a tennis ball moving 6 meter per second.
  • A joule it the amount of energy needed to life a medium tomato up one meter or is the energy release when dropping that same tomato from a height of one meter.
  • A joule is the amount of electricity needed to light a 1 W LED for one second.


  • International Bureau of Weights and Measures (2006). The International System of Units (SI) (8th ed.), p. 120. ISBN 92-822-2213-6.
  • Ristinen, Robert A.; Kraushaar, Jack J. (2006). Energy and the Environment (2nd ed.). Hoboken, NJ: John Wiley & Sons. ISBN 0-471-73989-8.