In quantum physics, what is the exact SI value of the Planck constant?

The exact SI value of the Planck constant is 6.62607015 × 10⁻³⁴ joule-seconds.

Planck’s constant, written h, relates a photon’s energy to its frequency through E = hf. It sets the scale at which quantum effects become important and appears throughout quantum mechanics, including wave mechanics and uncertainty relations.

The value became exact when the revised International System of Units took effect on 20 May 2019. Instead of defining the kilogram with a physical artifact, the SI now defines it by fixing the numerical value of h.

The reduced Planck constant, written ℏ and pronounced “h-bar,” equals h divided by 2π. It is common in angular-momentum equations and the Schrödinger equation. The elementary charge and the speed of light are also exact SI-defined constants.