The fundamental constant equal to the ratio of the energy of a quantum of energy to its frequency. It is defined as having a value of exactly 6.626 07015 × 10−34 Js. Because the Planck constant has been measured very accurately, it is now (since May 2019) used in the definition of the kilogram. In quantum-mechanical calculations the rationalized Planck constant (or Dirac constant) ħ = h/2π = 1.054 57180 × 10−34 Js is frequently used.