Kilograms

The kilogram, symbol kg, is the SI base unit of mass. It is defined by fixing the numerical value of Planck's constant h to exactly 6.62607015×10^-34 joule seconds. The kilogram is realized in practice through experiments such as the Kibble balance, which relate mass to fundamental constants.

Historically, the kilogram was defined by a platinum–iridium international prototype kilogram (IPK) kept at the International Bureau of Weights and Measures in Sèvres, France, and used as the standard for nearly 130 years. In 2019, this artifact-based definition was replaced.

The kilogram forms the basis for mass in the SI system. 1 kilogram equals 1000 grams; 1 gram equals one-thousandth of a kilogram. In common usage, 1 pound avoirdupois is approximately 0.45359237 kilograms. A liter of water at 4 °C has a mass of about 1 kilogram.

In science, engineering, and commerce, the kilogram is used to measure mass and is linked to international standards for traceability. National metrology institutes maintain calibrated masses and reference standards that are periodically compared to the SI definition.

Etymology: The term kilogram derives from the metric prefixes kilo- and -gram, meaning thousand grams. The SI unit name is kilogram, with the symbol kg; the plural remains kilogram or kilograms depending on usage.