Tonne: the metric ton and its different names
A tonne is exactly 1,000 kilograms, the unit called a metric ton in the United States. The name distinguishes it from the short ton and long ton, which have different masses despite their similar names. A tonne is approximately 2,204.6 pounds, 1.102 short tons, or 0.984 long tons. Its corresponding SI unit is the megagram, written Mg. That name expresses precisely the same mass, although it is less commonly used. The terminology can therefore identify either the same quantity under different names or different quantities under nearly identical names, depending on which term appears.
The official symbol for the tonne is the lowercase t. The CIPM adopted it with the unit in 1879, and the United States National Institute of Standards and Technology also uses it. Because t is a symbol rather than an abbreviation, it does not take a following period. Capitalization matters: T means tesla, MT means megatesla, and mT means millitesla. The combinations Mt and mt instead indicate megatonne and millitonne. These differences give a single letter's case a specific job in identifying the unit, with different arrangements pointing to mass or magnetic measurement.
In American usage, NIST recommends the name metric ton. An unqualified ton in the United States usually means a short ton of 2000 pounds, or 907.2 kilograms, and less often a long ton of 2240 pounds, or 1016 kilograms. Both tonne and metric ton are acceptable in Canadian English. In American and British English, tonne is usually pronounced like ton, though a pronunciation with the final e sounded also exists. Australian English commonly uses a different vowel. The spelling alone identifies the metric quantity more clearly than the shared usual pronunciation does.
Both ton and tonne descend from a Germanic word used around the North Sea since the Middle Ages for a large cask, or tun. Related forms include Old English and Old Frisian tunne, Medieval Latin tunna, and German and French tonne. A full tun standing about a meter tall could readily weigh a tonne. The modern spelling predates the introduction of SI in 1960: France had used tonne in this mass sense since 1842. Earlier American names included the French words millier and tonneau, but those terms became obsolete as names for the unit.
The older French tonne introduces another historical distinction. Before metrification, it meant 2000 French pounds, known as livres, and weighed 979 kilograms. The English ton weighed 1016 kilograms by comparison. The French word tonneau had preceded tonne for that quantity. Modern conversion rules are more explicit. A tonne equals 1000000 grams, while a short ton is exactly 0.90718474 tonnes and a long ton exactly 1.0160469088 tonnes. The megagram symbol Mg also needs its capital letter: mg means milligram. Similar spellings thus cross both historical systems and modern orders of magnitude.
A metric ton unit introduces a specialized trading meaning rather than another simple synonym. In metal trading, especially in the United States, mtu can refer to 10 kilograms. It traditionally described a tonne of ore containing 1 percent metal. Tungsten concentrates use metric tonne units to measure tungsten oxide quantities in 10-kilogram units; one such unit of tungsten VI contains 7.93 kilograms of tungsten. Uranium trading can use MTU differently, to mean a metric ton of uranium, equivalent to 1000 kilograms. The material being traded and the particular convention determine what the letters designate.
Tonne-based language also appears in energy measurement. A tonne of TNT serves as a proxy for energy, especially for explosions, and is approximately equivalent to 4.2 gigajoules. Kilotonne, megatonne, and gigatonne are used in this context, often as approximate figures. The petroleum industry's tonne of oil equivalent represents the energy released by burning a tonne of crude oil, approximately 42 gigajoules, although several slightly different definitions exist. Its energy is around ten times that of a tonne of TNT because atmospheric oxygen participates. Here a mass name helps express an energy quantity rather than the mass of a shipment.
The metric ton also gave rise to an older force term, tonne-force, equivalent to about 9.8 kilonewtons. That unit is now obsolete, but it has often been called simply tonne without an explicit indication that force is intended. There is another compact convention in the symbol kt: it means kilotonne, yet it is also the standard aviation symbol for the knot, a speed used for aircraft and watercraft. The standard knot symbol is kn. A lowercase t, a capital M, or the context of a force or speed can change the interpretation of a short technical label.