A kilogram was equal to the heft of this aging hunk of metal, and this cylinder, by definition, weighed exactly a kilogram. 2 THE DEFINITION of the kilogram has changed forever, 130 years after it was first established. In 1889, the 1 st General Conference of Weights and Measures (GCPM) adopted the International Prototype Kilogram (IPK) as the definition for the unit, which has remained unchanged until now. Time for a change. After the kilogram’s definition is changed officially — on May 20, 2019, also known as World Metrology Day — most people will never notice the difference, Wood said. (UPDATE: On May 20, 2019, the changes to the International System of Units officially went into effect, including new definitions for the kilogram, the … But that’s now changed. The unit of mass, the kilogram, is the last SI measurement unit defined by a physical artefact, rather than a mathematical constant. The new definition of the kilogram will be based on the Planck constant. Others call it “Le Grand K.” It is the one true kilogram in the world, upon which all measures of mass on Earth depend. On Friday, representatives from 60 countries decided to redefine the kilogram, setting its weight in terms of electric current rather than by comparison to a set object. Scientists have voted to redefine the kilogram, replacing the old physical definition with one based on unchanging constants of nature. The official definition of a kilogram is currently determined as the mass of a metal cylinder called the International Prototype of the Kilogram, but that could be about to change Greg L 2 / 2 Monday marks World Metrology Day, and this year’s edition sees a big change in the way the kilogram unit is defined. From today the kilogram is defined using the Planck constant, something that doesn’t change from quantum physics. Thus, the international community of measurement scientists has developed a new, more stable definition of the kilogram — one that is based on a constant of nature. It's impossible say which, since Big K, by definition, is exactly one kilogram. Even if someone took a file and shaved off a corner, Big K would still weigh one kilogram… A small cylinder called Le Grand K has defined the kilogram for more than a hundred years. But if a scratch ever rendered it lighter, the definition of the kilo literally shifted.