Planck constant

The Planck constant (denoted h) is a fundamental physical constant from quantum mechanics, equal to 6.62607015 × 10⁻³⁴ joule-seconds exactly. It appears in the energy of a photon (E = hν), the uncertainty principle, and the de Broglie wavelength.

Since 2019, the Planck constant defines the kilogram in the SI system. The 2019 SI redefinition moved several base units (kilogram, ampere, kelvin, mole) from dependence on physical artefacts to dependence on fundamental constants. Before 2019, the kilogram was defined by a platinum-iridium cylinder stored in a vault outside Paris (the “Kilogramme des Archives”); after 2019, it’s defined via Planck’s constant and laboratory-realizable apparatus called a Kibble balance.

The practical effect for everyday measurements: nothing. The new definition was chosen so that 1 kg under the new SI equals 1 kg under the old SI to within measurement precision. Your kitchen scale doesn’t care.