The defining measure for a kilogram is "Le Grand K", a cylinder of platinum and iridium held in Paris.
(The heavily guarded lump of metal is about the size of a plum. It is the only object known to science that has a mass of exactly 1kg, so it is the reference object from which the unit of mass is derived. This means that all objects measured in kilograms, whether a bag of sugar or an aircraft carrier, are defined by Le Grand K's mass.)