The Planck constant (h) is 6.626068 × 10-34 m2 kg/s. The relation of a photon's energy to frequency is
E = h × f
(E being energy, f being frequency).
The reduced Planck constant (or Dirac constant, abbreviated ℏ) is h/2π, which is convenient because many equations incorporate this expression.
A set of units based upon fundamental constants, including the Planck constant determine some minimal scales regarding current physical laws: Planck length, Planck mass, and Planck time. For example, a black hole of the Planck mass (a Planck hole) is considered as small as they can be.
tP ≡ √(ℏG/c5) mP ≡ √(ℏc/G) lP ≡ √(ℏG/c3)