A tonne (t) or metric ton, also referred to as a metric tonne or tonne métrique, is a measurement of mass equal to 1,000 kilograms. It is not an SI unit but is accepted for use with the SI. The proper SI unit for a tonne would be a "megagram" (Mg, see SI prefix), but this term is rarely used in practice. Though the spelling tonne predates the introduction of the SI system in 1960 (it has been used in France for centuries, where it comes from), it is now used as the standard spelling for the metric mass measurement in some English-speaking countries. In the United States the correct term is metric ton. The comparable imperial and US customary units are spelled ton in English.
In the USA this unit was defined in 1866 as a millier or a tonneau (both French words). This measure was used in Europe centuries earlier. However, neither of these latter words are in use in the USA and though they still appear in the statute, they have been declared obsolete by NIST.
The femtotonne shares the same symbol (ft) as the foot. The fractional multipliers are rarely used with the tonne unit basis, as the gram is both more fundamental and uses more familiar scaling factors. Hence 10 kilograms is more common than 10 millitonnes, or 5 ng rather than 5 ft.
The spelling tonne is from Gallic and 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 most liquids amounts to as many kilograms.
One tonne is equivalent to:
- One megagram (exactly); symbol Mg
- This is the official SI term, but not generally used in industry, in shipping nor colloquially
- 1000/0.45359237 pounds (exactly by definition), giving approximately
- 2204.622 621 848 775 807 lb (to 19 significant digits)
- 2204.622 622 lb (to ten significant digits)—an easy-to-remember figure
- 2205 lb (to four significant digits)
- 98.44% of a long ton
- One long ton (2240 lb) is 101.605% of a tonne
- 110.25% of a short ton
- One short ton (2000 lb) is 90.72% of a tonne
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). 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. 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, though it remains to be seen the impact of the "America COMPETES Act" of August 92007 which amended the Metric Act of 1866's obsolete definition of the metric system and its tables of units and to explicitly express preference for SI instead. 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 unit
A metric ton unit (MTU) can mean 10 kg 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.
- Section 4.1 of The International System of Units (SI), 8th Edition, 2006
- "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 
- Act of July 28, 1866, codified in 15 U.S.C. §205 
- Decimal multiples and submultiples of the unit of mass , BIPM - CIPM, 1967
- Metric System of Measurement: Interpretation of the International System of Units for the United States. See corrections in the Errata section of .
- America COMPETES Act. Public Law 110-69/39th Congress, H.R. 596. August 92007.
- Platt's Metals Guide to Specifications
- How Many? A Dictionary of Units of Measurement
- Y-12 National Security Complex: Browse Acronyms
- NRC Collection of Abbreviations (NUREG-0544, Rev. 4), UNITED STATES NUCLEAR REGULATORY COMMISSION
- NIST Special Publication 811, Guide for the Use of the International System of Units (SI)
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