A
tonne (unit symbol t) or
metric ton (U.S.
), also referred to as a metric
tonne, is a measurement of mass equal
to , or exactly the mass of one cubic metre of water at four degrees Celcius. It is not a unit in the
International System of Units
(SI), but is accepted for use with the SI. In SI units and
prefix, the tonne is a
megagram
(
Mg), but this usage is rare. 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
| Multiple |
Name |
Symbol |
|
Multiple (SI) |
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 |
| 1021 |
zettatonne |
Zt |
1027 |
(none) |
(none) |
| 1024 |
yottatonne |
Yt |
1030 |
(none) |
(none) |
|
|
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 has its origin in
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
- pounds (exactly by definition),
giving approximately
- 2205 lb (to four significant digits)
- 98.42% 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
Explanation
The unit symbol for the tonne 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
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