Kilogram

Kilogram is the base unit of mass in the International System of Units (SI), the current metric system, having the unit symbol kg.

It is widely used in science, engineering, and commerce worldwide, and is often simply called a kilo in everyday speech.

Definition

The kilogram was originally defined in 1795 as the mass of one litre of water. This was a practical definition, but difficult to use in practice. In 1889, a new definition was adopted, based on a physical artifact, the International Prototype of the Kilogram (IPK). This was a cylinder of platinum-iridium, kept at the International Bureau of Weights and Measures (BIPM) near Paris.

In 2019, the definition was changed again, to be based on fundamental physical constants. The change was made because the mass of the IPK had been observed to change slightly over time.

Kilogram and multiples

The kilogram is the only SI base unit with an SI prefix as part of its name. The gram (1/1000 of a kilogram) was the base unit of mass in the original French metric system. The kilogram, not the gram, is used in the definition of the SI system because the kilogram is more practical for the weights typically used in practice.

Realisation of the kilogram

In everyday commerce, the mass of a product is often measured and labelled in kilograms, where the kilogram is realised by a weighing scale. In science and engineering, where greater precision is often needed, the kilogram is realised using a Kibble balance or other experiment, in combination with the current definition and other physical constants.

See also

• Gram
• Metric system
• International System of Units
• International Bureau of Weights and Measures
• Kibble balance

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  International Prototype of the Kilogram
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  Unit relations in the new SI
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  Watt balance, large view