kilogramstandard

The kilogram standard is the SI unit of mass, symbol kg, and one of the seven base units in the International System of Units. It defines the mass of objects and serves as the reference for all mass measurements worldwide.

Historically, the standard was embodied by the International Prototype Kilogram (IPK), a platinum–iridium cylinder kept at the International Bureau of Weights and Measures (BIPM) near Paris. Since 1889, the IPK has served as the master reference against which other masses were calibrated, via a set of official copies called standard kilograms.

In 2019, the SI redefined the kilogram by fixing the numerical value of Planck’s constant h at exactly 6.62607015×10^-34 joule seconds. This change means the kilogram is no longer tied to a particular physical artifact; instead, it is realized through experiments that relate mass to fundamental constants. The primary realizations include the Kibble balance, which measures electrical power linked to Planck's constant, and methods based on counting atoms, such as silicon lattice spheres used to determine the Avogadro constant.

The International Prototype Kilogram is still maintained as a physical artifact and used for comparison and continuity checks, but it no longer defines the unit. National metrology institutes realize the kilogram in practice by adopting methods tied to h and through calibrated artifacts and reference materials.

The redefinition provides greater long-term stability and universality for mass measurements, enabling precise science and technology without reliance on a single artifact.