The kilogram (also kilogramme) is the base unit of mass in the International System of Units (SI), the metric system, having the unit symbol kg. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo colloquially.

General information
Unit systemSI base unit
Unit ofmass
1 kg in ...... is equal to ...
   Avoirdupois   2.204622 pounds[Note 1]
   British Gravitational   0.0685 slugs

The kilogram was originally defined in 1795 as the mass of one litre of water. Modern superseding definitions of a kilogram agree with this original definition to within 30 parts per million. In 1799, the platinum Kilogramme des Archives replaced it as the standard of mass. In 1889, 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.[1] 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.[2] 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, although the IPK and other precision kilogram masses remain in use as secondary standards for all ordinary purposes.