English: A graph of the relative change in mass of selected kilogram prototypes. From The Third Periodic Verification of National Prototypes of the Kilogram (1988—1992), G. Girard, Metrologia 31 (1994) 317—336. The International Prototype Kilogram (IPK) kept in Sevres, France, and the other national prototype kilogram standards have been found to vary in mass over the years. Since the IPK defines the kilogram, the only way to detect changes in mass is to compare the mass of the prototypes, which is done every 40 years. This graph shows the "mass drift" of the numbered prototypes compared to the IPK.

• The IPK is represented by the stylized K in an old-English typeface.
• No. 39, which lost 665 µg relative to the IPK, had originally been allocated to Japan in 1894. In 1958, following the Second World War, it was ceded to the Republic of Korea.
• No. 23 was allocated to Finland in 1890.
• No. 8(41) was accidentally stamped with the number 41, but its accessories carry the proper number 8: since there is no prototype marked 8, this prototype is referred to as 8(41).