The metric unit for mass is the gram (g) . The gram is the basic unit of mass in the metric system, and it is defined as the mass of water that fills a 1 cm³ cube. Other common metric units of mass include the kilogram (kg) and the tonne (t) . The kilogram is equal to 1000 grams, while the tonne is equal to 1000 kilograms. The metric system uses prefixes to express weight as multiples or fractions of 1 gram. For example, a milligram (mg) is 1/1000 of a gram, while a kilogram (kg) is 1000 grams.