International Prototype of the Kilogram (IPK)

The former International Prototype of the Kilogram (IPK) is an artefact whose mass defined the SI unit of mass until the implementation of a revised definition of the kilogram on 20 May 2019.

The IPK has been conserved at the BIPM since 1889, when it was sanctioned by the 1st General Conference on Weights and Measures (CGPM). It is of cylindrical form, with diameter and height of about 39 mm, and is made of an alloy of 90 % platinum and 10 % iridium. Initially the IPK had two official copies; over the years, one official copy has been replaced and four others have been added, so that there are now six official copies.

Access to the IPK and its official copies is under the strict supervision of the CIPM.


kg definition - BUTTON

selected publications - TITLE

Selected publications