The international prototype of the kilogram, an artefact made of platinum-iridium, is kept at the BIPM under the conditions specified by the 1st CGPM in 1889 when it sanctioned the prototype and declared:
This prototype shall henceforth be considered to be the unit of mass.
The 3rd CGPM (1901), in a declaration intended to end the ambiguity in popular usage concerning the use of the word "weight", confirmed that:
The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.
The complete declaration appears here.
It follows that the mass of the international prototype of the kilogram is always 1 kilogram exactly, m() = 1 kg. However, due to the inevitable accumulation of contaminants on surfaces, the international prototype is subject to reversible surface contamination that approaches 1 µg per year in mass. For this reason, the CIPM declared that, pending further research, the reference mass of the international prototype is that immediately after cleaning and washing by a specified method (PV, 1989, 57, 104-105 and PV, 1990, 58, 95-97). The reference mass thus defined is used to calibrate national standards of platinum-iridium alloy (Metrologia, 1994, 31, 317-336).
The symbol, m(), is used to denote the mass of the international prototype of the kilogram, .