Origin of SI units - Tonne

The tonne  (SI symbol: t) is a non-SI metric unit of mass equal to 1000 kilograms; it is thus equivalent to one megagram (Mg). 1000 kilograms is equivalent to 0.984 tons. Although not part of the SI, the tonne is "accepted for use with" SI units and prefixes by the CIPM, along with several other units like the bar, litre and day.

In French and all English-speaking countries that are predominantly metric, tonne is the correct spelling in writing. It is usually pronounced the same as ton /tʌn/, but when it is important to clarify that the metric term is meant, rather than ton (imperial unit), the final "e" can also be pronounced /ˈtʌnɪ/. In Australia, it is also pronounced /tɒn/.

Before metrication in the UK the unit used for most purposes was the Imperial ton of 2,240 pounds avoirdupois , equivalent to 1,016 kg, differing by just 1.6% from the tonne. The UK Weights and Measures Act 1985 explicitly excluded from use for trade many units and terms, including the ton and the term "metric tonne". However, for many purposes the Imperial ton and the tonne are so similar that it is not important to distinguish them, even in writing, and the spelling "ton" is still often used where "tonne" is meant. For example, even the Guinness Book of World Records accepts metrication without marking this by changing the spelling.

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 tunna, German and French tonne) to designate a large cask, or tun. A full tun, standing about a metre high, could easily weigh a tonne. An English tun (an old wine cask volume measurement equivalent to 954 litres) of wine weighs roughly a tonne, 954 kg if full of water, a little less for wine.

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 units comparable to the tonne are both spelled ton in English, though they differ in mass.

The SI symbol for the tonne is "t", adopted at the same time as the unit itself in 1879. It is a symbol, not an abbreviation, and should not be followed by a period. Informal and non-approved symbols or abbreviations include "T", "mT", "MT", "mt", and "Te" (particularly in the offshore and nuclear industries).[citation needed] Some of these are actually SI symbols for other units: "T" is the SI symbol for the tesla and "Mt" is the SI symbol for megatonne (equivalent to one teragram); if describing TNT equivalent units of energy, this is equivalent to 4.184 petajoules.

A metric ton unit (MTU) can mean 10 kg within metal (e.g. tungsten, manganese) trading. It traditionally referred to a tonne 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 1000 kg.

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, approximately 42 GJ. There are several slightly different definitions.

A gigatonne of Carbon dioxide equivalent (GtCO2eq) is a unit used by the UN climate change panel, IPCC, to measure the effect of a technology or process on global warming.

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.

In this post we introduced tonne, and the difference between ton and tonne. Please use the correct words when talking these two units.