The tonne ( or ; symbol: t) is a unit of mass equal to 1000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton to distinguish it from the non-metric units of the short ton (United States customary units), and the long ton (British imperial units). It is equivalent to approximately 2204.6 pounds, 1.102 short tons, and 0.984 long tons. The official SI unit is the megagram (symbol: Mg), a less common way to express the same mass. Symbol and abbreviations The BIPM symbol for the tonne is t, adopted at the same time as the unit in 1879. Its use is also official for the metric ton in the United States, having been adopted by the United States National Institute of Standards and Technology (NIST). It is a symbol, not an abbreviation, and should not be followed by a period. Use of minuscule letter case is significant, and use of other letter combinations can lead to ambiguity. For example, T, MT, mT, Mt and mt are the SI symbols for the tesla, megatesla, millitesla, megatonne (one teragram), and millitonne (one kilogram), respectively. If describing TNT equivalent units of energy, one megatonne of TNT is equivalent to approximately 4.184 petajoules. Origin and spelling In English, tonne is an established spelling alternative to metric ton. In the United States and United Kingdom, tonne is usually pronounced the same as ton (), but the final "e" can also be pronounced, i.e. "tunnie" (). In Australia, the common and recommended pronunciation is . 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 and to a lesser extent to a long ton of , the tonne is rarely used in speech or writing. Both terms are acceptable in Canadian usage. 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 , German and French tonne) to designate a large cask, or tun. A full tun, standing about a metre high, could easily weigh a tonne. The spelling tonne pre-dates the introduction of the SI in 1960; it has been used with this meaning in France since