Learn more about Tonne
- This article deals with the metric tonne. For the imperial ton, see Ton.
A tonne or metric ton (symbol t), sometimes referred to as a metric tonne, is a measurement of mass equal to 1,000 kilograms. It is not an SI unit but is accepted for use with the SI. Though the spelling tonne predates the introduction of the SI system in 1960, it is now used as the standard spelling for the metric mass measurement in English. The similar Imperial units and United States customary units are spelled ton in English.
This unit was defined in the United States in 1866<ref> Act of July 28, 1866, codified in 15 U.S.C. §205 </ref> as a millier or a tonneau. However, neither of these are in use and though they still appear in the statute, they have been declared obsolete by NIST.<ref>"Metric System of Measurement: Interpretation of the International System of Units for the United States", Federal Register notice of July 28, 1998, 63 F.R. 40333 </ref>
1 tonne is defined as 1000 kilograms or 1 megagram.
The spelling tonne is from French. In Old English the spelling was tunne. The various spellings and meanings (tonne, ton, tun) derive from the late Latin tunna, "cask" - a full cask about a metre high could easily weigh a tonne. It may also be of Celtic origin.
One tonne is equivalent to:
- 1 megagram (exactly). Symbol Mg. This is the SI term
- 1000/0.45359237 pounds (exactly) ~ 2205 pounds (by comparison the imperial measurements long ton and short ton are 2200 and 2000 pounds, respectively)
The official symbol is t. T and mT and mt (especially in the combination mmt for "million metric tons" compare to Mt for megatonne) are also sometimes 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 1000th (though in practice fractional prefixes aren't generally used with the tonne).
In France and the English-speaking countries that are predominantly metric, the spelling tonne is widespread. However, in Britain, the ton used prior to metrication was the long ton of 2240 pounds (approximately 1016 kg). This is so close to the tonne that many 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 2000 lb (about 907 kg).
Like grams and kilograms, tonnes gave rise to a (now obsolete) force unit of the same name: 1 tonne-force = 9.80665 kilonewtons (kN), 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 which is accepted for use with SI; the tonne-force or metric ton-force is not acceptable for use with SI.
 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 1000 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.
 Metric ton in popular culture
- In the 1999 movie, Austin Powers: The Spy Who Shagged Me, villain Dr Evil makes a reference to character Fat Bastard's weight by saying, "He weighs a metric ton".
- Metric Ton is also the name of a hardcore band from Jacksonville, Fl.
 See also
- Units using the tonne
- Other tons
- Conversion of units
- Orders of magnitude (mass)
- NIST Special Publication 811, Guide for the Use of the International System of Units (SI)
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