The tonne (SI unit symbol: t) is a metric system unit of mass equal to 1000 kilograms. It is a non-SI unit accepted for use with SI. To avoid confusion with the ton, it is also known as the metric tonne and metric ton in the United States and occasionally in the United Kingdom. In SI units and prefixes, the tonne is a megagram (Mg), a rarely-used symbol, easily confused with mg, for milligram.

In the U.S., the National Institute of Standards and Technology supports the symbol "t" for the tonne. The abbreviation "mt" or "MT" is also used in the U.S.

The SI unit symbol for the tonne is "t". Abbreviations for "tonne" ("metric ton" and "metric tonne") include "T", "mT", "MT", and "mt" (especially in the combination "mmt" or "MMT" for "million metric tons"), and "Te" (particularly in the offshore and nuclear industries).^{[citation needed]} SI has no abbreviations for SI units, only symbols, and some of the abbreviations in use are actually SI symbols for other units: "T" is the SI symbol for the tesla and "Mt" is the SI symbol for megatonne.