Kilogram (kg)

The kilogram is the SI base unit of mass. It is currently defined by the mass of the International Prototype Kilogram.

Accurately measuring the mass of an object in kilograms is essential for almost everything, from administering the optimum dose of a drug to correctly manufacturing materials with the desired properties.
In the International Bureau of Weights and Measures in France, there is a platinum-iridium alloy cylinder called the International Prototype Kilogram. Since 1884, this has defined what we call a kilogram.

Redefinition

Currently, we compare the gravitational force on an object with the gravitational force on a reference piece of metal known as a 'standard weight'. The standard weight is in turn compared with the International Prototype Kilogram. However, this may have changed since it was produced in 1884, but we have no way of knowing. Contamination, cleaning or just time may have increased or decreased it.

Under the expected redefinition, in May 2019 we will compare the gravitational force on an object with a magnetic force using a Kibble balance. A kilogram will be therefore be defined using the Planck constant, a fixed numerical constant which will not change over time.