A
tonne (
t) or
metric ton (
U.S.),
also referred to as a
metric tonne, is a measurement of
mass equal to , or approximately the mass of one cubic metre of
water. It is not an
SI unit but is accepted for use with the SI. 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 with this meaning in France since 1842), 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, but these terms are now obsolete.
The
Imperial and
US customary units comparable to the tonne are both spelled
ton in English.
Derived units
Multipliers are never used to denote fractions of a 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
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:
- *This is the official SI term, but not generally used in industry, in shipping nor colloquially
- pounds (exactly by definition), giving approximately
- *2205 lb (to four significant digits)
- *One long ton (2240 lb) is 101.605% of a tonne
- *One short ton (2000 lb) is 90.72% of a tonne
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 (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. In the U.S. an unqualified mention of a "ton" almost invariably refers to a
short ton of .
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 (an exact decimal multiple of the SI unit of mass, the kilogram) which is accepted for use with SI: 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.
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 1,000 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.
In the case of
uranium, the acronym 'MTU' is sometimes considered to be 'metric ton of uranium', meaning 1,000 kg.
See also
