The kilogram is getting an overhaul.
'The Big K' is a lump of metal kept away under glass in Paris that has defined the measurement for more than a hundred years.
But the metal is finally showing its age so this World Metrology day will be its last. It's being fixed with the help of kiwi metrologists.
To tell us more about the change is one of those kiwis, Fleur Francois, director of New Zealand's national metrology institute, Measurement Standards Laboratory.