SI definition of the kilogram

The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.

The dimension of mass is [M], its unit is the kilogram and its symbol is kg.

The International Prototype Kilogram

The original definition (1795) of the kilogram was a mass equal to that of a cubic decimeter of pure air-free water at the temperature of melting ice (273.15 K). This was altered four years later to the mass of water in the same volume but at the temperature at which water has its maximum density (which occurs at 277.13 K; Kaye & Laby, 1959). An all-platinum prototype with the same mass as the cubic decimeter of water was manufactured the same year and designated the Kilogramme des Archives. The current standard kilogram mass, a cylindrical platinum–iridium alloy, made in 1879 and accepted as the standard since 1889, is known as the International Prototype Kilogram, (IPK). It is now the only SI standard which is a manufactured artifact. The IPK and six replicas are stored at BIPM in a controlled environment. Further copies, known as replicas, were manufactured for distribution to other national metrology laboratories throughout the world.

The stability of the International Prototype Kilogram

As the IPK has, by definition, a mass of one kilogram, it has a zero measurement error. However, when the mass of the IPK is compared with the masses of the replicas, the IPK is apparently losing mass relative to all of them.