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The tonne (SI symbol: t) is a metric system unit of mass equal to 1,000 kilograms (2,204.6 pounds), or 1 Mg (megagram). It is a non-SI unit accepted for use with SI. To avoid confusion with short ton, it is also known as metric ton in the United States.
The SI symbol for the tonne is "t". It is also the symbol supported by the US National Institute of Standards and Technology. Non-approved abbreviations for tonne and "metric ton" include "T", "mT", "MT", and "mt", and "Te" (particularly in the offshore and nuclear industries). SI has no abbreviations for SI units, only symbols, and some of the abbreviations in use are actually SI symbols for other units: "T" is the SI symbol for the tesla and "Mt" is the SI symbol for megatonne.
In French and all English-speaking countries that are predominantly metric, tonne is the correct spelling in writing. It is usually pronounced the same as ton //, but when it is important to clarify that the metric term is meant, rather than short ton, the final "e" can also be pronounced //  In Australia, it is also pronounced //.
Before metrication in the UK the unit used for most purposes was the Imperial ton of 2,240 pounds avoirdupois (usually referred to as the long ton in the US), equivalent to 1,016 kg, differing by just 1.6% from the tonne. The UK Weights and Measures Act 1985 explicitly excluded from use for trade many units and terms, including the ton and the term "metric tonne". However, for many purposes the Imperial ton and the tonne are so similar that it is not important to distinguish them, even in writing, and the spelling "ton" is still often used where "tonne" is meant. For example, even the Guinness Book of World Records accepts metrication without marking this by changing the spelling. In the United States metric ton is the name for this unit used and recommended by NIST; an unqualified mention of a ton almost invariably refers to a short ton of 2,000 pounds (907 kg), and tonne is rarely used in speech or writing.
Ton and tonne are both derived from a Germanic word in general use in the North Sea area since the Middle Ages (cf. Old English and Old Frisian tunne, Old High German and Medieval Latin tunna, German and French tonne) to designate a large cask, or tun. A full tun, standing about a metre high, could easily weigh a tonne. An English tun (an old wine cask volume measurement equivalent to 954 litres) of wine weighs roughly a ton (of any type), 954 kg if full of water, a little less for wine.
The spelling tonne pre-dates the introduction of the SI in 1960; it has been used with this meaning in France since 1842, when there were no metric prefixes for multiples of 106 and above, and is now used as the standard spelling for the metric mass measurement in most English-speaking countries. In the United States, the unit was originally referred to using the French words millier or tonneau, but these terms are now obsolete. The Imperial and US customary units comparable to the tonne are both spelled ton in English, though they differ in mass.
One tonne is equivalent to:
For multiples of the tonne, it is more usual to speak of, for example, millions of tonnes; kilotonne, megatonne, and gigatonne are more usually used for the energy of nuclear and other events, used loosely as approximate figures, with little need to distinguish between metric and other tons, spelling either megatons or megatonnes.
A metric ton unit (MTU) can mean 10 kilograms (22 lb) within metal (e.g. tungsten, manganese) trading, particularly within the US. It traditionally referred to a metric ton of ore containing 1% (i.e. 10 kg) of metal.
In the petroleum industry the tonne of oil equivalent (toe) is a unit of energy: the amount of energy released by burning one tonne of crude oil, approximately 42 GJ. There are several slightly different definitions.
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 thermochemical calorie per milligram). Hence, 1 kt TNT = 4.2 TJ, 1 Mt TNT = 4.2 PJ.
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. In contrast to the tonne as a mass unit, 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.