metric ton

A tonne (t) or metric ton (U.S.),^[1] also referred to as a metric tonne, is a measurement of mass equal to 1,000 kg or 2,204.62262 lb, or approximately the mass of one cubic metre of water. It is not an SI unit but is accepted for use with the SI.^[2] If prefixes were used completely consistently, the SI unit for a tonne would be a megagram, or Mg (see SI prefix), but this term is rarely used. The spelling tonne pre-dates the introduction of the SI system in 1960 (it has been used in France for about two and a half centuries),^{[citation needed]} 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,^[3] but these terms are now obsolete.^[1] The Imperial and US customary units comparable to the tonne are both spelled ton in English.

Derived units

Only the units in the first column of the above table are actually used. The multipliers denoting fractions of a tonne are never used, as the fundamental SI unit of mass is the kilogram and not the tonne. Hence a mass of 10,000 g would normally be referred to as 10 kilograms (kg), and not 10 millitonnes.

Origin

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 many commonly used liquids (aqueous solutions) approximates to as many kilograms.

Conversions

One tonne is equivalent to:

• One megagram (exactly);
  • 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
• 110.23% of a short ton
  • One short ton (2000 lb) is 90.72% of a tonne
• 32,150.75 troy ounces

Explanation

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 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 2,240 pounds (1,020 kg) (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.^[4] In the U.S. an unqualified mention of a "ton" almost invariably refers to a short ton of 2,000 pounds (907.1847 kg).

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 which is accepted for use with SI: the tonne-force or metric ton-force is not acceptable for use with SI, as the SI unit of force is 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 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.

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.^[5][6]

In the case of uranium, the acronym 'MTU' is sometimes considered to be 'metric ton of uranium', meaning 1000 kg.^{[7][8][9][10]}

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