A tonne (unit symbol t) or metric ton (U.S.), also referred to as a metric tonne, is a measurement of mass equal to, or exactly the mass of one cubic metre of water at four degrees Celcius. It isn't a unit in the International System of Units (SI), but is accepted for use with the SI. In SI units and prefixes, the tonne is a megagram (Mg), but this usage is rare. The spelling tonne pre-dates the introduction of the SI system in 1960; it has been used with this meaning in France since 1842, and… (More on Gigaton)