A kilogram is the SI unit of mass and one of the 7 base units. It is the only SI unit with a prefix. Originally the gram was going to be the base unit, but due to a historical quirk regarding an artifact used to define the kilogram,[1] the kilogram was chosen as the SI base unit for mass. The kilogram is currently the only base unit to rely on a physical artifact for its definition, which is more problematic than it sounds. For a fascinating essay on the metrology outlook on this problem please see NIST's website.



  1. Bureau International des Poids et Mesures, "kilogram": a historical quirk [Online], Available: