Given the task definition and input, reply with output. In this task, we ask you to elaborate the sentence without changing its general meaning. You can do so by explaining further the input sentence, using more precise wording, adding qualifiers and auxiliary information etc.

The kilogram is the base unit of mass in the International System of Units ( SI ) .

The kilogram ( also kilogramme ) is the base unit of mass in the metric system , formally the International System of Units ( SI ) , having the unit symbol kg .