For 121 years it has provided a physical basis for scientific definitions of mass and energy.
But now scientists are losing faith with the one kilogram metallic artefact known as the "international prototype".
Experts meeting today at the Royal Society in London will hear about new research moving closer to the first non-physical definition of the kilogram.
If this is achieved – possibly within the next five years – it will mark a historic change, the scrapping of the last manufactured object on which fundamental units of measurement depend.
The international prototype is a small cylinder of plutonium-iridium alloy, just under four centimetres tall and wide, whose mass defines the kilogram.
Since 1889 it has remained triple-locked under two glass dome bell jars in a vault at the International Bureau of Weights and Measures (BIPM) on the outskirts of Paris.
However, measurements made over the last 100 years indicate it has lost weight – around 50 micrograms, the equivalent of a small grain of sand.