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A tonne (unit symbol t) or metric ton (U.S.marker), also referred to as a metric tonne, is a measurement of mass equal to , or exactly the mass of one cubic metre of water at four degrees Celcius. It is not a unit in the International System of Units (SI), but is accepted for use with the SI. In SI units and prefix, the tonne is a megagram (Mg), but this usage is rare. 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 U.S., the units were 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.

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 not generally used in industry, in shipping nor colloquially
  • pounds (exactly by definition), giving approximately
    • 2205 lb (to four significant digits)
  • 98.42% of a long ton
    • One long ton (2240 lb) is 101.605% of a tonne
  • 110.23% of a short ton
    • One short ton (2000 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 nuclear industry.

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 (approximately) 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 .

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. Prefixes are also used e.g. kilotonne, megatonne, gigatonne; especially for expressing nuclear weapon yield, based on a specific combustion energy of TNT of 4.184 MJ/kg (or one calorie—specifically a thermochemical calorie—per milligram). Hence, 1 kt TNT = 4.184 TJ, 1 Mt TNT = 4.184 PJ.

The SI unit of energy is the joule. Assuming that TNT contains 1,000 small (thermochemical) calories per gram (4.184 kJ/g), one tonne TNT is more correctly referred to as 4.184 gigajoules. It is usually used to describe the energy of explosions.

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.

In the case of uranium, the acronym MTU is sometimes considered to be metric ton of uranium, meaning 1,000 kg.

See also


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