The kilogram is the legal (SI) unit of mass (symbol: kg).
The mass of the international prototype kilogram in platinum iridium deposited in 1889 at the International Bureau for Weights and Measures (BIPM, in Sèvres, Hauts-de-Seine), the French representative of which is the LNE (Laboratoire national d'essai et de métrologie). There are other national prototypes in other countries. The international standard kept in France bears the number 35.
After the measurement made in 1889, two others were made to compare the international prototype with the national prototypes of other countries and with references in 1946 and 1989. These measurements revealed a modification in the mass of around 30 micrograms with a dispersion in the measurements of the order of 100 micrograms.
It is considered that the accuracy is of the order of 5x10-8, which is a poor value for a standard unit. Another way of producing a standard mass is currently being sought. One approach is to dematerialise the reference (as in the case of the metre and the second) using a mechanical and electromagnetic device called a watt balance.