The tonne (unit symbol t) or metric ton (U.S.), 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, 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. 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, 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. 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%.
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:
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. 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.
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.
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.