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. Monday marks World Metrology Day, and this year’s edition sees a big change in the way the kilogram unit is defined. But that’s now changed. Scientists have voted to redefine the kilogram, replacing the old physical definition with one based on unchanging constants of nature. 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. Time for a change. The new definition of the kilogram will be based on the Planck constant. 2 It's impossible say which, since Big K, by definition, is exactly one kilogram. A small cylinder called Le Grand K has defined the kilogram for more than a hundred years. Even if someone took a file and shaved off a corner, Big K would still weigh one kilogram… But if a scratch ever rendered it lighter, the definition of the kilo literally shifted. From today the kilogram is defined using the Planck constant, something that doesn’t change from quantum physics. THE DEFINITION of the kilogram has changed forever, 130 years after it was first established. Others call it “Le Grand K.” It is the one true kilogram in the world, upon which all measures of mass on Earth depend. 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. 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. 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 (UPDATE: On May 20, 2019, the changes to the International System of Units officially went into effect, including new definitions for the kilogram, the … A kilogram was equal to the heft of this aging hunk of metal, and this cylinder, by definition, weighed exactly a kilogram. The unit of mass, the kilogram, is the last SI measurement unit defined by a physical artefact, rather than a mathematical constant.