A tonne (t) or metric ton (U.S.), If prefixes were used completely consistently, the SI unit for a tonne would be a megagram, or Mg (see SI prefix), but this term's rarely used. The spelling tonne pre-dates the introduction of the SI system in 1960 (it's been used in France for about two and a half centuries), and's now used as the standard spelling for the metric mass measurement in most English-speaking countries. The comparable imperial and US customary units are spelled ton in English. In t… (More on Megagram)