The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.
The kilogram is the only SI base unit which is still realized by a prototype body – as about 200 years ago. The "original kilogram", the international prototype kilogram, is kept under a double "cheese-dish cover" in a laboratory of the International Bureau of Weights and Measures (Bureau International des Poids et Mesures, BIPM) in Sèvres near Paris. The international metrology institutes have copies of it. Each of the national prototypes is regularly compared with its international counterpart. However, the various national prototypes more and more deviate from one another. Scientists are, therefore, intensively looking for a method by which the unit of mass, too, can be traced back to a fundamental constant. One possibility is the NA. It gives the number of particles contained in one mole of a specific substance. In the case of the carbon nuclide most frequently occurring, i.e. 12C, the mole has a mass of 12 g. If it were possible to count the atoms very precisely (there are about 6.022 ·1023), an exact measure of the mass would be available. However, the counting accuracy must still be improved before the kilogram can be redefined.