|unit system||si base unit|
|1 kg in ...||... is equal to ...|
| avoirdupois|| ≈ 2.205 pounds[note 1] |
| british gravitational|| ≈ 0.0685 slugs |
the kilogram (also kilogramme) is the base unit of mass in the metric system, formally the international system of units (si), having the unit symbol kg. it is a widely used measure in science, engineering, and commerce worldwide, and is often simply called a kilo in everyday speech.
the kilogram was originally defined in 1795 as the mass of one litre of water. this was a simple definition, but difficult to use in practice. by the latest definitions of the unit, however, this relationship still has an accuracy of 30 ppm. in 1799, the platinum kilogramme des archives replaced it as the standard of mass. in 1879, a cylinder of platinum-iridium, the international prototype of the kilogram (ipk) became the standard of the unit of mass for the metric system, and remained so until 2019. the kilogram was the last of the si units to be defined by a physical artefact.
the kilogram is now defined in terms of the second and the metre, based on fixed fundamental constants of nature. this allows a properly-equipped metrology laboratory to calibrate a mass measurement instrument such as a kibble balance directly by measuring natural phenomena, with no need to use an artefact.