A tonne (t) or metric ton, also referred to as a metric tonne or tonne métrique, is a measurement of mass equal to 1,000 kilograms, or 2204.6226 pounds. It is not an SI unit but is accepted for use with the SI. The proper SI unit for a tonne would be a "megagram" (Mg, see SI prefix), but this term is rarely used in practice. Though the spelling tonne predates the introduction of the SI system in 1960 (it has been used in France for about two and a half centuries, where it comes from), it is now used as the standard spelling for the metric mass measurement in some English-speaking countries. In the United States the correct term is metric ton. The comparable imperial and US customary units are spelled ton in English.
In the U.S. this unit was defined in 1866 as a millier or a tonneau (both French words). This measure was used in Europe centuries earlier; the millier was formerly 1000 livres (489.5 kg) and the tonneau was a wine cask size equivalent to the tun, containing ~904.8 L. However, neither of these latter words is in use in the U.S. and though they still appear in the statute, they have been declared obsolete by NIST.
The fractional multipliers are rarely used with the tonne unit basis, as the gram is both more fundamental and uses more familiar scaling factors. Hence 10 kilograms is more common than 10 millitonnes, or 5 ng rather than 5 ft.
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 U.S. 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.
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.
If the metal is uranium, the acronym 'MTU' is sometimes considered to be 'metric ton of uranium' i.e. 1000 kg.