# Kilogram

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.

kilogram | |
---|---|

General information | |

Unit system | SI base unit |

Unit of | mass |

Symbol | kg |

Conversions | |

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.