The Planck constant (h) is 6.626068 × 10-34 m2 kg/s. The relation of a photon's energy (E) to frequency (F) is:
E = h × f
The reduced Planck constant (aka Dirac constant, abbreviated ℏ) is h/2π, which is convenient because many equations incorporate this expression. A set of units based upon the Planck constant and other fundamental units (Planck units) are fully based upon physical constants, i.e., a type of natural units. Planck units reflect some minimal scales regarding the regime of 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)