Kilogram
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The kilogram (alternative spelling kilogramme) is the base unit of mass in the International System of Units (SI), having the unit symbol kg. It means 'one thousand grams'. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo colloquially.
Definition
The kilogram is defined in terms of the second and the metre, both of which are based on fundamental physical constants. This allows a properly equipped metrology laboratory to calibrate a mass measurement instrument such as a Kibble balance as the primary standard to determine an exact kilogram mass.
The kilogram was originally defined in 1795 during the French Revolution as the mass of one litre of water. The current definition of a kilogram agrees with this original definition to within 30 parts per million.
The revised definition for kg is in terms of the Planck constant, the speed of light and hyperfine transition frequency of 133^{Cs} as approved by the General Conference on Weights and Measures (CGPM) on November 16, 2018.