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A tonne (t) or metric tonne (also referred to as a metric ton), is a measurement of mass equal to 1000 kg, or almost exactly the mass of one cubic metre of pure water at 3.98 °C.[1] It is not an SI unit but is accepted for use with the SI.[2]

Using the SI prefixes, the correct SI terminology for a tonne would be a megagram (Mg) but this term is not often used. In most countries, tonne is used as the standard terminology for the metric mass measurement.


There does not appear to be a definitive consensus as to the origin or etymology of the word tonne.[3][4][5][6] Most sources indicate that it has a Celtic or Gaelic origin that led to the Latin word tunna and to the words tun and tonne in French and English. In the evolution of both of those languages, tun and tonne originally referred to a barrel or cask with a volume of about 954 litres (almost a cubic metre), which for wine, beer and many commonly used aqueous solutions would weigh about a tonne.


One tonne equals:

  • Exactly 1 megagram (1 Mg)
  • Exactly 1000 kilograms
  • Exactly 1 metric tonne (or 1 metric ton)
  • 2204.62 pounds (to 6 significant digits)
  • 0.98421 long ton (to 5 significant digits)
  • 1.1023 short ton (to 5 significant digits)

Other usage of the word tonne

The word tonne is also used to express quantities other than mass, such as:

  • A tonne of coal equivalent (TCE) ≈ 29.31 GJ (varies from one coal to another)
  • A tonne of oil equivalent (TOE) ≈ 41.87 GJ (varies from one petroleum crude oil to another)
  • A tonne of TNT equivalent = 4.184 GJ
  • A tonne of refrigeration = 13,898 kJ/h = 3.861 kW
    • A ton of refrigeration = 12,000 Btu/h = 3.517 kW (A ton of refrigeration is defined slightly differently than a tonne of refrigeration)


  1. The density of air-free water at a pressure of 1 atmosphere and 3.98 °C is 999.974 kg per cubic metre.   September 2001, The International Association for the Properties of Water and Steam Guideline on the Use of Fundamental Physical Constants and Basic Constants of Water
  2. Section 4.1 of The International System of Units (SI) (PDF), 8th Edition, 2006
  3. tonne The American Heritage Dictionary of the English Language, Fourth Edition, 2000
  4. Eric Partridge (1966). Origins: an etymological dictionary of modern English, 4th Edition. Routledge. ISBN 0-415-050-744. 
  5. ton Online Etymology Dictionary
  6. tonne The Free Dictionary