Tonne

For the units of mass, force, and other quantities in general, see Ton.

A tonne (symbol t), sometimes referred to as a metric tonne, is a measurement of mass. It is not an SI unit but is accepted for use with the SI. Though the spelling tonne predates the introduction of the SI system in 1960, it is now used as the standard spelling for the metric mass measurement in English. The similar Imperial units and United States customary units are spelled ton in English.

Definition

1 tonne is defined as 1000 kilograms or 1 megagram.

Multiples

Multiple	Name	Symbol	Multiple	Name	Symbol
10⁰	tonne	t
10¹	decatonne	dat	10^–1	decitonne	dt
10²	hectotonne	ht	10^–2	centitonne	ct
10³	kilotonne	kt	10^–3	millitonne	mt
10⁶	megatonne	Mt	10^–6	microtonne	µt
10⁹	gigatonne	Gt	10^–9	nanotonne	nt
10¹²	teratonne	Tt	10^–12	picotonne	pt
10¹⁵	petatonne	Pt	10^–15	femtotonne	ft
10¹⁸	exatonne	Et	10^–18	attotonne	at
10²¹	zettatonne	Zt	10^–21	zeptotonne	zt
10²⁴	yottatonne	Yt	10^–24	yoctotonne	yt

Origin

The spelling tonne is from French. In Old English the spelling was tunne. The various spellings and meanings (tonne, ton, tun) derive from the late Latin tunna, "cask" - a full cask about a metre high could easily weigh a tonne. It may also be of Celtic origin.

Conversions

One tonne is equivalent to:

1 megagram (exactly). Symbol Mg. This is the SI term
1000/0.45359237 pounds (exactly) ~ 2205 pounds

Explanation

The official symbol is t. T and mT and mt (especially in the combination mmt for "million metric tons") are also sometimes 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 1000th.)

In France and the English-speaking countries that are predominantly metric, the spelling tonne is widespread. However, in Britain, the ton used prior to metrication was the long ton of 2240 pounds (approximately 1016 kg). This is so close to the tonne that many 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 US an unqualified mention of a "ton" almost invariably refers to a short ton of 2000 lb (about 907 kg).

Like grams and kilograms, tonnes gave rise to a (now obsolete) force unit of the same name: 1 tonne-force = 9.80665 kilonewtons (kN), 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.

M is SI standard for millon or 1e6 and Metric tonne being t, this would make millon metric tonnes as Mt in SI nomenclature.

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.

References

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

Units of mass

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