Kilogram
Until 2019 a kilogram was defined as having a mass equal to a platinum alloy cylinder, the International Prototype Kilogram (informally Le Grand K or IPK), held by the International Bureau of Weights and Measures (BIPM) in France. In 2019 it became defined in terms of the Planck constant, by taking the fixed numerical value of the Planck constant h to be 6.62607015×10-34 when expressed in the unit J·s, which is equal to kg·m2·s-1, where the metre and the second are defined in terms of c and ??Cs.
