tonne

The tonne ( or ; symbol: t) is a metric unit of mass equal to 1,000 kilograms. It is also referred to as a metric ton. It is equivalent to approximately pounds; , and 0.984 long tons (UK). The official SI unit is the megagram (symbol: Mg), a less common way to express the same mass.

Symbol and abbreviations

The BIPM symbol for the tonne is t, adopted at the same time as the unit in 1879.Table 6
. BIPM. Retrieved on 2011-07-10. Its use is also official for the metric ton in the United States, having been adopted by the United States National Institute of Standards and Technology (NIST). It is a symbol, not an abbreviation, and should not be followed by a period. Use of

minuscule

letter case is significant, and use of other letter combinations can lead to ambiguity. For example, T, MT, mT, Mt and mt are the SI symbols for the tesla, megatesla, millitesla, megatonne (one teragram), and millitonne (one kilogram) respectively. If describing TNT equivalent units of energy, one megatonne of TNT is equivalent to approximately 4.184 petajoules.

Origin and spelling

In English, ''tonne'' is the established spelling alternative to ''metric ton''. In the US and UK, tonne is usually pronounced the same as ton (), but the final "e" can also be pronounced, i.e. "tunnie" (). In Australia, the common and recommended pronunciation is . In the United States, ''metric ton'' is the name for this unit used and recommended by NIST;Metric System of Measurement: Interpretation of the International System of Units for the United States
(PDF). See corrections in the Errata section o

. an unqualified mention of a ''ton'' almost invariably refers to a short ton of , and ''tonne'' is rarely used in speech or writing. Both terms are acceptable in Canadian usage.

''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 ,

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 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.

Conversions

One tonne is equivalent to:
*In kilograms: by definition.
*In grams: or 1 megagram (Mg). Megagram is the corresponding official SI unit with the same mass. Mg is distinct from mg, milligram.
*In pounds: Exactly pounds (lb) by definition of the pound, or approximately .
*In short tons: Exactly  short tons (ST), or approximately  ST.
**One short ton is exactly .National Institute of Standards and Technology.
*In long tons: Exactly  long tons (LT), or approximately  LT.
**One long ton is exactly .

A tonne is the mass of one cubic metre of pure water: at 4 °C one thousand

litre

s of pure water has an absolute mass of one tonne.

Derived units

For multiples of the tonne, it is more usual to speak of thousands or millions of tonnes. Kilotonne, megatonne, and gigatonne are more usually used for the energy of nuclear explosions and other events in equivalent mass of TNT, often loosely as approximate figures. When used in this context, there is little need to distinguish between metric and other tons, and the unit is spelt either as ''ton'' or ''tonne'' with the relevant prefix attached.

^*The equivalent units columns use the short scale large-number naming system currently used in most English-language countries, e.g. 1 billion = 1,000 million = 1,000,000,000.

^†Values in the equivalent short and long tons columns are

rounded

to five significant figures. See Conversions for exact values.

^ǂThough non-standard, the symbol "kt" is also used (instead of the standard symbol "kn") for knot, a unit of speed for aircraft and sea-going vessels, and should not be confused with kilotonne.

Alternative usage

A metric ton unit (mtu) can mean 10 kg (approximately 22 lb) within metal (e.g.

tungsten

,

manganese

) trading, particularly within the US. It traditionally referred to a metric ton of ore containing 1% (i.e. 10 kg) of metal.
The following excerpt from a mining geology textbook describes its usage in the particular case of tungsten:

"Tungsten concentrates are usually traded in metric tonne units (originally designating one tonne of ore containing 1% of WO₃, today used to measure WO₃ quantities in 10 kg units. One metric tonne unit (mtu) of tungsten (VI) contains 7.93 kilograms of tungsten." (Walter L Pohl, ''Economic Geology: Principles and Practices'', English edition, 2011, p 183.)

In the case of

uranium

, ''MTU'' is sometimes used in the sense of ''metric ton of uranium'' (1,000 kg).NRC Collection of Abbreviations (NUREG-0544, Rev. 4), United States Nuclear Regulatory Commission
Nrc.gov (2011-03-13). Retrieved on 2011-07-10.

A gigatonne is a unit of mass often used by the coal mining industry to assess and define the extent of a coal reserve.

Use of mass as proxy for energy

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 t TNT = approx. 4.2  GJ, 1 kt TNT = approx. 4.2  TJ, 1 Mt TNT = approx. 4.2  PJ.

The SI unit of energy is the

joule

. Assuming that a TNT explosion releases 1,000 small (thermochemical) calories per gram (approx. 4.2 

kJ

/g), one tonne of TNT is approx. equivalent to 4.2

gigajoules

.

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, approx, 42 GJ. There are several slightly different definitions. This is ten times as much as a tonne of TNT because atmospheric oxygen is used.

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

* Metre–tonne–second system of units
* Orders of magnitude (mass)
* Ton
** Tonnage
** Ton (volume)

Notes and references

External links

*NIST Special Publication 811
''Guide for the Use of the International System of Units (SI)''

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Non-SI metric units
Units of mass