Kilogram

The kilogram is the SI unit of mass.

At the end of the 18th century, a kilogram was the mass of a cubic decimeter of water. In 1889, the 1st CGPM sanctioned the international prototype of the kilogram, made of platinum-iridium, and declared: This prototype shall henceforth be considered to be the unit of mass.

The 3d CGPM (1901), in a declaration intended to end the ambiguity in popular usage concerning the word "weight," confirmed that:

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

The kilogram, rather than the gram, is the unit of mass due to historical accident. An early version of the metric system proposed the "grave", equal to the mass of one cubic decimeter of water, as the unit of mass. However, after the French revolution, a unit equal to one-thousandth of the "grave" was chosen, as most common commercial transactions were for weights less than one "grave". Due to the small mass of the gram, it was decided to prepare a prototype artifact of 1000 grams, or one kilogram, as the mass standard.

References

• The name "kilogram": a historical quirk, Bureau International des Poids et Mesures.
• Unit of mass (kilogram), SI brochure, Section 2.1.1.2, Bureau International des Poids et Mesures.