What material is the International Prototype of the Kilogram made of?

The International Prototype of the Kilogram was made of a platinum–iridium alloy. Known as Pt10Ir, it contained 90% platinum and 10% iridium by mass.

The alloy was selected because platinum is exceptionally resistant to oxidation and corrosion, while iridium greatly increases hardness. Together, the metals produce a dense, stable material with low magnetic susceptibility. The prototype was machined as a right circular cylinder about 39 millimetres in diameter and height, a shape chosen to reduce surface area and limit possible contamination.

The object, often nicknamed Le Grand K, served as the international reference for the kilogram from 1889 until 2019. It and official copies were kept by the International Bureau of Weights and Measures near Paris and used to calibrate national mass standards. It was not made of pure platinum: an earlier standard, the Kilogramme des Archives, used pure platinum, but that metal was too soft for the most durable standard.

In 2019, the kilogram was redefined using the fixed numerical value of the Planck constant rather than a physical artifact. The platinum–iridium prototype remains historically important, but it no longer defines the SI kilogram.