The kilogram or kilogramme (symbol: kg) is the base unit of mass in the International System of Units (SI). Since 20 May 2019, it has been defined in terms of fundamental physical constants. Prior to 20 May 2019, it was defined by a platinum alloy cylinder, the International Prototype Kilogram (informally Le Grand K or IPK), manufactured in 1889, and carefully stored in Saint-Cloud, a suburb of Paris.


Kibble balance from NIST is a used to determine the value of the planck constant which defines the mass of the kilogram. image: wikipedia


The kilogram was originally defined as the mass of a litre (cubic decimetre) of water. That was an inconvenient quantity to precisely replicate, so in 1799 a platinum artefact was fashioned to define the kilogram. That artefact, and the later IPK, remained the standard of the unit of mass for the metric system until 20 May 2019.


A replica of the prototype kilogram on display at Cité des Sciences et de l'Industrie, featuring the protective double glass bell. image: Japs 88 /wikipedia

The prototype changed ever so slightly

In spite of best efforts to maintain it, the IPK has diverged from its replicas by approximately 50 micrograms since their manufacture late in the 19th century. This led to efforts to develop measurement technology precise enough to allow replacing the kilogram artifact with a definition based directly on physical phenomena. This definition was adopted in 2019.

The new definition is based on invariant constants of nature, in particular the Planck constant, which was changed to being defined rather than measured, thereby fixing the value of the kilogram in terms of the second and the metre, and eliminating the need for the IPK. 

The new definition was approved by the General Conference on Weights and Measures (CGPM) on 16 November 2018.

 The Planck constant relates a light particle's energy, and hence mass, to its frequency. The new definition only became possible when instruments ( a Kibble Balance) were devised to measure the Planck constant with sufficient accuracy based on the IPK definition of the kilogram.

Source adapted from: Wikipedia contributors. (2019, May 21). Kilogram. In Wikipedia, The Free Encyclopedia. Retrieved 07:42, May 21, 2019, from