Kilogram is the SI base unit for mass. It is equal to the mass of the international prototype of the kilogram, a platinum-iridium standard kept at the International Bureau of Weights and Measures. The symbol for kilogram is kg. SI units derived from it include grams, centigrams and milligrams.
According to The National Institute of Standards and Technology (NIST), each country that subscribed to the International Metric Convention was assigned a copy of the international standards; these are known as National Prototypes. In the United States, the national prototype for the kilogram is a platinum-iridium cylinder kept at the NIST.