Tonne

This article is about the metric unit of mass. For other ton units, see Ton. For other uses, see Tonne (disambiguation).

The tonne, known as the metric ton in the US (SI unit symbol: t, U.S. abbreviation: "mt"), often put as "metric tonne" to avoid confusion with ton, is a metric system unit of mass equal to 1000 kilograms. The tonne is not an International System of Units (SI) unit, but is accepted for use with the SI. In SI units and prefixes, the tonne is a megagram (Mg).

Unit symbol

The SI unit symbol for the tonne is "t". "T", "mT" and "mt" (especially in the combination "mmt" for "million metric tons") are also used, and "Te" (particularly in the offshore and nuclear industries) is sometimes used.^{[citation needed]} These alternative abbreviations or symbols are similar to SI terms: "T" is the SI unit symbol for the tesla, "Mt" for megatonne and "m" is used as an SI prefix for "milli" - the 0.001 multiplier.

Origin and spelling

In France and the English-speaking countries that are predominantly metric, the spelling tonne is widespread. This is generally true in the United Kingdom; however, the ton used prior to metrication is equivalent to 2,240 pounds (1,016 kg) (in the US this ton is usually referred to as the long ton) 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 US an unqualified mention of a ton almost invariably refers to a short ton of 2,000 pounds (907 kg).

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. The old English wine cask volume measurement known as a 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.

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. Pronunciation of tonne (the word used in the United Kingdom) and ton is usually identical.

Conversions

One tonne is equivalent to:

• One megagram (exactly);
  • megagram is the official SI term, but generally not used in industry or shipping, nor colloquially
• ¹⁰⁰⁰⁄_{0.453 592 37} pounds (exactly by definition), giving approximately
  • 2205 lb (to four significant digits)
• 98.42% of a long ton
  • One long ton (2,240 lb) is 101.605% of a tonne
• 110.23% of a short ton
  • One short ton (2,000 lb) is 90.72% of a tonne

Derived units

Tonnes			Grams
Multiple	Name	Symbol	Multiple	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

Alternate usage

A metric ton unit (MTU) can mean 10 kilograms (22 lb) 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.

Use of mass as proxy for energy

Main article: TNT equivalent

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.

The SI unit of energy is the joule. Assuming that a TNT explosion releases 1,000 small (thermochemical) calories per gram (4.2 kJ/g), one tonne of TNT is equivalent to 4.2 gigajoules.

Unit of force

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.

See also

• Ton
  • Short ton
  • Long ton
  • Tonnage
  • Ton (volume)

• Metre-tonne-second system of units
• Tonne of oil equivalent
• Orders of magnitude (mass)

References

• NIST Special Publication 811, Guide for the Use of the International System of Units (SI)