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. Conversions One tonne is equivalent to: In kilograms: by definition. In grams: or 1 megagram (Mg). Megagram is the corresponding official SI unit with the same mass. Mg is distinct from mg, milligram. In pounds: Exactly pounds (lb) by definition of the pound, or approximately . In short tons: Exactly short tons (ST), or approximately ST. One short ton is exactly . In long tons: Exactly long tons (LT), or approximately LT. One long ton is exactly . A tonne is the mass of one cubic metre of pure water: at 4 °C one thousand litres of pure water has an absolute mass of one tonne. Derived units As a non-SI unit, the use of SI metric prefixes with the tonne does not fall within the SI standard. 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 in equivalent mass of TNT, 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 spelled 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 = 1000 million = . †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 used (instead of the standard symbol "kn") for knot, a unit of speed for aircraft and sea-going vessels, and should not be confused with kilotonne. Alternative usages Metric ton units A metric ton unit (mtu) can mean 10 kg (approximately 22 lb) within metal trading, particularly within the United States. It traditionally referred to a metric ton of ore containing 1% (i.e. 10 kg) of metal. The following excerpt from a