The kilogram (kg)

Quantity, unit, symbol and definition  

The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.

Note: This international prototype is made of platinum-iridium and is kept at the International Bureau of Weights and Measures, Sèvres, Paris, France; the British copy (No. 18) is kept at NPL.

 

Realisation and use at The National Physical Laboratory

 Kilogram masses and sub-multiples of 1 kg, made from similar materials, may be compared on the NPL precision balance to about 1 microgram.