metric tonne
Massexact by definition
The metric tonne is exactly 1000 kilograms. It is not an SI unit but is accepted for use with the SI, and it is exact because it is a pure decimal multiple of the kilogram. In North American writing it is often called the metric ton to distinguish it from the short ton.
Watch out: Three different tons are in circulation and they differ by up to 12 percent. The metric tonne is 1000 kg, the US short ton is 2000 lb (907.18474 kg), and the UK long ton is 2240 lb (1016.0469088 kg). Commodity prices, shipping quotes and emissions figures are frequently misconverted between them.
| 1 t | 1,000 kg |
The name comes from the tun, a large wine cask, which is also the ancestor of the shipping tonnage that measures a vessel's capacity rather than its mass.
About the metric tonne
The tonne is the unit where sloppy conversion costs real money. Write "1000 tons of steel" without saying which ton and you have left a 10 percent ambiguity in the order: 1000 metric tonnes is 1 000 000 kg, 1000 US short tons is 907 185 kg, and 1000 UK long tons is 1 016 047 kg. The convention that helps is spelling: many style guides use tonne for the metric 1000 kg and ton for the imperial ones, but the convention is not universal and North American technical writing often says "metric ton" in full precisely because it cannot trust the spelling to survive editing.
Two derived usages are worth knowing. Carbon accounting is done in tonnes of CO₂ equivalent, so international climate figures are metric throughout. And a "ton" of air conditioning is not a mass at all but a rate of heat removal, 12 000 BTU per hour, inherited from the ice trade. Context, not the word, tells you which quantity is meant.