The British tonne (/tʌn/) (British and SI; SI symbol: t), which is the same as the metric ton in the United States, is a non-SI metric unit of mass equal to 1,000 kilograms; or one megagram (Mg); it is equivalent to approximately 2,204.6 pounds, 1.10 short tons (US) or 0.984 long tons (imperial). Although not part of the SI per se, the tonne is "accepted for use with" SI units and prefixes by the International Committee for Weights and Measures, along with several other units like the bar, litre and day.
Symbol and abbreviations
The SI symbol for the tonne is "t", adopted at the same time as the unit itself in 1879. Its use is also official, for the metric ton, within the United States, having been adopted by the US National Institute of Standards and Technology. 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", and "mt". Some of these are 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.
Origin and spelling
In French and all English-speaking countries that are predominantly metric, tonne is the correct spelling. It is usually pronounced the same as ton /tʌn/, but when it is important to clarify that the metric term is meant, rather than short ton, the final "e" can also be pronounced, i.e. "tonny" /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 (usually referred to as the long ton in the US), 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 certain imperial units, including the ton, unless the item being sold or the weighing equipment being used was weighed or certified prior to 1 December 1980, and even then only if the buyer was made aware that the weight of the item was measured in imperial units.
In the United States metric ton is the name for this unit used and recommended by NIST; an unqualified mention of a ton almost invariably refers to a short ton of 2,000 pounds (907 kg), and tonne is rarely used in speech or writing.
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 and US customary units comparable to the tonne are both spelled ton in English, though they differ in mass.
One tonne is equivalent to:
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, 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 sometimes used for knot, a unit of speed for sea-going vessels, and should not be confused with kilotonne.
A metric ton unit (MTU) can mean 10 kilograms (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.
In the case of uranium, the acronym MTU is sometimes considered to be metric ton of uranium, meaning 1,000 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.
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 kt TNT = 4.2 TJ, 1 Mt TNT = 4.2 PJ.
The SI unit of energy is the joule. Assuming that a TNT explosion releases 1,000 small (thermochemical) calories per gram (4.2 kJ/g), one tonne of TNT is equivalent to 4.2 gigajoules.
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.