The tonne (SI unit symbol: t) is a metric system unit of mass equal to 1000 kilograms. It is a non-SI unit accepted for use with SI.^[1][2] To avoid confusion with the ton, it is also known as the metric tonne and metric ton in the United States^[3] and occasionally in the United Kingdom.^[4] In SI units and prefixes, the tonne is a megagram (Mg), a rarely-used symbol, easily confused with mg, for milligram.^[5]

In the U.S., the National Institute of Standards and Technology supports the symbol "t" for the tonne.^[6] The abbreviation "mt" or "MT" is also used in the U.S.^[7]

Symbol and abbreviations[link]

The SI unit symbol for the tonne is "t". Abbreviations for "tonne" ("metric ton" and "metric tonne") include "T", "mT", "MT", and "mt" (especially in the combination "mmt" or "MMT" for "million metric tons"), and "Te" (particularly in the offshore and nuclear industries).^{[citation needed]} SI has no abbreviations for SI units, only symbols, and some of the abbreviations in use are actually SI symbols for other units: "T" is the SI symbol for the tesla and "Mt" is the SI symbol for megatonne.

Origin and spelling[link]

In France and the English-speaking countries that are predominantly metric, tonne is the usual formal usage in writing, usually pronounced the same as ton, /tʌn/, but with the final "e" pronounced (/ˈtʌnɪ/) when it is important to clarify that the metric, rather than Imperial, term is meant^[8]. Before metrication in the UK the unit used for most purposes was the Imperial ton of 2,240 pounds avoirdupois (usually referred to as the long ton in the US), equivalent to 1,016 kg, differing by just 1.6% from the tonne. The UK Weights and Measures Act 1985 explicitly excluded from use for trade many units and terms, including the ton and the term "metric ton"^[9]. However, for many purposes the Imperial and metric tons are so similar that it is not important to distinguish them, even in writing, and the long-standing spelling "ton" continues to be widely used where strictly speaking "tonne" is meant. For example, even the Guinness Book of World Records accepts metrication without marking this by changing the spelling^{[citation needed]}. In the United States metric ton is the name for this unit used and recommended by NIST;^[6] an unqualified mention of a ton almost invariably refers to a short ton of 2,000 pounds (907 kg), and tonne is rarely used in speech or writing.

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.^[10] A full tun, standing about a metre high, could easily weigh a tonne. An English tun (an old wine cask volume measurement equivalent to 954 litres) of wine weighs roughly a ton (of any type), 954 kg if full of water, a little less for wine.

The spelling tonne pre-dates the introduction of the SI in 1960; it has been used with this meaning in France since 1842,^[11] 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.^{[12][13][14][15]} In the United States, the unit was originally referred to using the French words millier or tonneau,^[16] but these terms are now obsolete.^[3] The Imperial and US customary units comparable to the tonne are both spelled ton in English, though they differ in mass.

Conversions[link]

One tonne is equivalent to:

• One megagram (by definition);
  • megagram, Mg, is the official SI term, but rarely used; Mg is easily mistaken to be mg, milligram
• ¹⁰⁰⁰⁄_{0.453 592 37} pounds (exactly, by definition of the pound),^[17]
  • approximately 2205 lb
• 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[link]

For multiples of the tonne, it is more usual to speak of, for example, millions of tonnes; kilotonne, megatonne, and gigatonne are more usually used for the energy of nuclear and other events, used loosely as approximate figures, with little need to distinguish between metric and other tons, spelling either megatons or megatonnes^[18].

Alternate usage[link]

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.^[19][20]

In the case of uranium, the acronym MTU is sometimes considered to be metric ton of uranium, meaning 1,000 kg.^{[21][22][23][24]}

In the petroleum industry the tonne of oil equivalent (toe) is a unit of energy: the amount of energy released by burning one tonne of crude oil, approximately 42 GJ. There are several slightly different definitions.

Use of mass as proxy for energy[link]

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[link]

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[link]

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

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

References[link]

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

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