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From Wikipedia, the free encyclopedia

The tonne (unit symbol t) or metric ton (U.S.),[1] also referred to as a metric tonne, is a unit of mass equal to 1,000 kg (2,204.62262 lb) or approximately the mass of one cubic metre of water at four degrees Celsius. It is sometimes abbreviated as mt in the United States,[2] but this conflicts with other SI symbols. The tonne is not a unit in the International System of Units (SI), but is accepted for use with the SI.[3] In SI units and prefixes, the tonne is a megagram (Mg). The spelling tonne pre-dates the introduction of the SI system in 1960; it has been used with this meaning in France since 1842,[4] and is now used as the standard spelling for the metric mass measurement in most English-speaking countries.[5][6][7][8] In the United States, the unit was originally referred to using the French words millier or tonneau,[9] but these terms are now obsolete.[1] The Imperial and US customary units comparable to the tonne are both spelled ton in English, though they differ in mass. Pronunciation of tonne (the word used in the UK) and ton is usually identical, but is not too confusing unless accuracy is important as the tonne and UK long ton differ by only 1.6%.


Derived units

Multiple Name Symbol Multiple (SI) Name Symbol
100 tonne t 106 megagram Mg
103 kilotonne kt 109 gigagram Gg
106 megatonne Mt 1012 teragram Tg
109 gigatonne Gt 1015 petagram Pg
1012 teratonne Tt 1018 exagram Eg
1015 petatonne Pt 1021 zettagram Zg
1018 exatonne Et 1024 yottagram Yg
1021 zettatonne Zt 1027 (none) (none)
1024 yottatonne Yt 1030 (none) (none)

Multipliers are never used to denote fractions of a tonne. Hence a mass of 10,000 g would normally be referred to as 10 kilograms (kg), and not 10 millitonnes.


The spelling tonne has its origin in French. The term applied to the barrel of the largest size. In Old English the spelling was tunne, "cask" — a full cask about a metre high could easily weigh a tonne. The antiquated British wine cask volume measurement tun is close to a metric tonne in weight as it defines about 954 litres which for many commonly used liquids (aqueous solutions) approximates to as many kilograms.


One tonne is equivalent to:

  • One megagram (exactly);
    • This is the official SI term, but generally not used in industry or shipping, nor colloquially
  • 10000.45359237 pounds (exactly by definition), giving approximately
    • 2205 lb (to four significant digits)
  • 98.42% of a long ton
    • One long ton (2,240 lb) is 101.605% of a tonne
  • 110.23% of a short ton
    • One short ton (2,000 lb) is 90.72% of a tonne


The unit symbol for the tonne is t. T and mT and mt (especially in the combination mmt for million metric tons compare to Mt for megatonne) are also occasionally used, but all of these are deprecated since they conflict with internationally agreed SI symbols. (T is the SI symbol for the tesla and m is SI prefix 'milli', meaning 0.001.) Te is also sometimes used, particularly in the offshore and nuclear industries.

In France and the English-speaking countries that are predominantly metric, the spelling tonne is widespread. This is generally true in Britain; however, the ton used prior to metrication was the long ton of 2,240 pounds (1,016.0469 kg) and this is so close to the tonne that some people draw little distinction and continue to use the old spelling. For example, even the Guinness Book of World Records accepts metrication without marking this by changing the spelling. For the United States, metric ton is the name for this unit used and recommended by NIST.[10] In the U.S. an unqualified mention of a ton almost invariably refers to a short ton of 2,000 pounds (907.1847 kg).

Like the gram and the kilogram, the tonne gave rise to a (now obsolete) force unit of the same name, the tonne-force, equivalent to about 9.8 kilonewtons: a unit also often called simply "tonne" or "metric ton" without identifying it as a unit of force. Note that it is only the tonne as a unit of mass (an exact decimal multiple of the SI unit of mass, the kilogram) which is accepted for use with SI: the tonne-force or metric ton-force is not acceptable for use with SI, partly because it is not an exact multiple of the SI unit of force, the newton.

Use of mass as proxy for energy

The tonne of trinitrotoluene (TNT) is used as a proxy for energy, usually of explosions (TNT is a common high explosive). Prefixes are used: kiloton(ne), megaton(ne), gigaton(ne), especially for expressing nuclear weapon yield, based on a specific combustion energy of TNT of about 4.2 MJ/kg (or one calorie—specifically a thermochemical calorie—per milligram). Hence, 1 kt TNT = 4.2 TJ, 1 Mt TNT = 4.2 PJ.

The SI unit of energy is the joule. Assuming that a TNT explosion releases 1,000 small (thermochemical) calories per gram (4.2 kJ/g), one tonne of TNT is equivalent to 4.2 gigajoules.

Alternate usage

A metric ton unit (MTU) can mean 10 kg (22.046226 pounds) within metal (e.g. tungsten, manganese) trading, particularly within the USA. It traditionally referred to a metric ton of ore containing 1% (i.e. 10 kg) of metal.[11][12]

In the case of uranium, the acronym MTU is sometimes considered to be metric ton of uranium, meaning 1,000 kg.[13][14][15][16]

See also


  1. ^ a b "Metric System of Measurement: Interpretation of the International System of Units for the United States" (PDF). Federal Register 63 (144): 40333–40340. July 28, 1998. 63 FR 40333. 
  2. ^ See, for example, NASA Human Spaceflight Commission Final Report: Seeking a Human Spaceflight Program Worthy of a Great Nation, October 2009, Review of U.S. Human Spaceflight Plans Committee, p. 65-66.
  3. ^ Section 4.1 of The International System of Units (SI) (PDF), 8th Edition, 2006
  4. ^ Source: TLF French dictionary
  5. ^ "Guidance Note on the use of Metric Units of Measurement by the Public Sector". National Measurement Office. 2007. Retrieved 2010-02-13.  "Tonne" is listed under "The Principal Metric Units of Measurement" on p. 7.
  6. ^ "National Measurement Regulations 1999". Australian Government. 1999. Retrieved 2010-02-13.  "Tonne" is listed under Schedule 1, Part 3 as a non-SI unit of measurement used with SI units of measurement.
  7. ^ "Appendix 4: Units of Measurement and Conversion Factors". MAF (Ministry of Agriculture and Forestry (New Zealand)). Retrieved 2010-02-13. 
  8. ^ "Canada Gazette". Government of Canada. 1998-2007. Retrieved 2010-02-13. "The Corporation shall pay to producers selling and delivering wheat produced in the designated area to the Corporation the following sums certain per tonne basis..." 
  9. ^ Act of July 28, 1866, codified in 15 U.S.C. § 205
  10. ^ Metric System of Measurement: Interpretation of the International System of Units for the United States (PDF). See corrections in the Errata section of [1].
  11. ^ Platt's Metals Guide to Specifications
  12. ^ How Many? A Dictionary of Units of Measurement
  13. ^ Radioactive Waste Management Profiles — Acronyms, Abbreviations, and Glossary
  14. ^ "Glossary". (June 2000). Disposition of Surplus Hanford Site Uranium, Hanford Site, Richland, Washington. US Department of Energy.
  15. ^ "Acronyms". Y-12 National Security Complex.
  16. ^ NRC Collection of Abbreviations (NUREG-0544, Rev. 4), United States Nuclear Regulatory Commission

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