With the introduction of the decimal metric system in the late eighteenth century, the kilogram—the fundamental unit of mass—was defined as the mass of one cubic decimeter (dm) of distilled water at 4° Celsius, the temperature at which water reaches maximum density. A new platinum-iridium prototype was produced in 1889, and has since served as the official definition of the kilogram mass.

The kilogram-force is a different unit, measuring the force exerted by terrestrial attraction on a one-kilogram mass. In common usage, the kilogram or kilo is the legal unit of weight equal to 1,000 grams.