The kilogram (kg) is the base unit of mass in the SI system. 1 kg = 1,000 g = 2.20462 lb. The kilogram is the only SI unit that is defined in terms of an artifact instead of an experiment, this artifact being a cylindrical platinum-iridium slug kept by the International Bureau of Weights and Measures at Sèvres, near Paris. It is copied by all the bureaus of standards and laboratories in the world which adhere to the SI. For example, a replica of the metal cylinder at Sèvres is kept at the National Institute of Standards and Technology in Gaithersburg, Maryland, and serves as the SI standard of mass for the United States.
The kilogram used to be defined as the mass of 1 liter of water. This mass served as the archetype for the artifact at Sèvres.