The erg is a unit of energy equal to 10−7 joules (100 nJ). It originated in the Centimetre–gram–second system of units (CGS). It has the symbol erg. The erg is not an SI unit. Its name is derived from ergon (ἔργον), a Greek word meaning 'work' or 'task'.[1]

Unit systemCGS units
Unit ofenergy
Derivation1 erg = 1 dyn⋅cm
1 erg in ...... is equal to ...
   CGS base units   1 cm2⋅g⋅s−2
   SI units   10−7 J
   British Gravitational System   7.375621×10−8 ft⋅lbf

An erg is the amount of work done by a force of one dyne exerted for a distance of one centimetre. In the CGS base units, it is equal to one gram centimetre-squared per second-squared (g⋅cm2/s2). It is thus equal to 10−7 joules or 100 nanojoules (nJ) in SI units.

  • 1 erg = 10−7 J = 100 nJ
  • 1 erg = 10−10 sn⋅m = 100 psn⋅m = 100 picosthène-metres
  • 1 erg = 624.15 GeV = 6.2415×1011 eV
  • 1 erg = 1 dyn⋅cm = 1 g⋅cm2/s2
  • 1 erg = 2.77778×10−11 W⋅h


In 1864, Rudolf Clausius proposed the Greek word ἐργον (ergon) for the unit of energy, work and heat.[2][3] In 1873, a committee of the British Association for the Advancement of Science, including British physicists James Clerk Maxwell and William Thomson recommended the general adoption of the centimetre, the gramme, and the second as fundamental units (C.G.S. System of Units). To distinguish derived units, they recommended using the prefix "C.G.S. unit of ..." and requested that the word erg or ergon be strictly limited to refer to the C.G.S. unit of energy.[4]

In 1922, chemist William Draper Harkins proposed the name micri-erg as a convenient unit to measure the surface energy of molecules[5] in surface chemistry.[6][7] It would equate to 10−14 erg,[5][8][9][10][11] the equivalent to 10−21 joule.

The erg has not been a valid unit since 1 January 1978[12] when the European Economic Community ratified a directive of 1971 that implemented the International System (SI) as agreed by the General Conference of Weights and Measures.[13] It is still widely used in astrophysics[14] and sometimes in mechanics.

See also

  • Foe (unit), relative measure for energy released by a supernova
  • Lumen second, for the lumerg and lumberg units
  • Metre–tonne–second system of units


  1. Goodell, Thomas Dwight (1889). The Greek in English (2nd ed.). Henry Holt and Company. p. 40.
  2. Clausius, Rudolf (1867). "Appendices to Sixth Memoir [1864]. Appendix A. On Terminology.". In Hirst, T. Archer (ed.). The Mechanical Theory of Heat, With Its Applications to the Steam-engine and to the Physical Properties of Bodies. London: J. Van Voorst. p. 253. Retrieved 2014-03-17. editions:PwR_Sbkwa8IC.
  3. Howard, Irmgard K. (2001). "S is for Entropy. U is for Energy. What Was Clausius Thinking?" (PDF). Journal of Chemical Education. 78 (4): 505. Bibcode:2001JChEd..78..505H. doi:10.1021/ed078p505. Retrieved 2014-03-17.
  4. Thomson, Sir W; Foster, Professor GC; Maxwell, Professor JC; Stoney, Mr GJ; Jenkin, Professor Fleeming; Siemens, Dr; Bramwell, Mr FJ (September 1873). Everett, Professor (ed.). First Report of the Committee for the Selection and Nomenclature of Dynamical and Electrical Units. Forty-third Meeting of the British Association for the Advancement of Science. Bradford: John Murray. p. 224. Retrieved 2014-03-17.
  5. Jerrard, H. G.; McNeill, D. B. (1993) [1963]. A Dictionary of Scientific Units - Including dimensionless numbers and scales (6 ed.). London: Chapman and Hall. p. 100. ISBN 0412467208. OCLC 803100353. OL 1351307M.
  6. Cardarelli, François (1999) [1966]. Scientific unit conversion: A practical guide to metrication (2 ed.). Springer-Verlag London Limited. doi:10.1007/978-1-4471-0805-4. ISBN 978-1-85233-043-9. 1447108051, 9781447108054. Retrieved 2015-08-25.
  7. Cardarelli, François (2003). Encyclopaedia of Scientific Units, Weights and Measures. Springer-Verlag London Ltd. ISBN 978-1-4471-1122-1.
  8. Roberts, Lathrop Emerson; Harkins, William Draper; Clark, George Lindenberg (2013-07-01) [1922]. The Orientation of Molecules in Surfaces, Surface Energy, Adsorption, and Surface Catalysis. V. The Adhesional Work Between Organic Liquids and Water: Vaporization in Steps as Related to Surface Formation. University of Chicago Digital Preservation Collection. University of Chicago. Retrieved 2015-08-25.
  9. Holmes, Harry N. (1925). Colloid Symposium Monograph - Papers Presented at the Second National Symposium on Colloid Chemistry, Northwestern University, June, 1924. Vol. 2. The Chemical Catalog Company, Inc. Retrieved 2015-02-15.
  10. "Journal of the American Chemical Society - Issues for 1898-1901 include Review of American chemical research, v. 4-7; 1879-1937, the society's Proceedings". Journal of the American Chemical Society. American Chemical Society. 44: 665. 1922. ISSN 0002-7863. Retrieved 2015-02-15.
  11. Partington, James Riddick (2010-02-17) [1949]. An Advanced Treatise on Physical Chemistry: Fundamental principles. The properties of gases. Vol. 1. Longmans, Green. Retrieved 2015-08-25.
  12. Neufert, Ernst; Neufert, Peter; Kister, Johannes (2012-03-26). Architects' Data. John Wiley & Sons. ISBN 9781405192538.
  13. Jennings, W. A. (October 1972). "SI units in radiation measurement". The British Journal of Radiology. 45 (538): 784–785. doi:10.1259/0007-1285-45-538-784. ISSN 0007-1285. PMID 5078949.
  14. "Are ergs commonly used in astrophysics? If so, is there a specific reason for it?". Physics Stack Exchange. 2016-02-12. Retrieved 2018-09-15.
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