The Planck constant (h) is
6.62607015 × 10-34 m2 kg/s
(m²kg/s is equivalent to joules × seconds).
This value is exact by design: the International System of Units (SI) now defines the relevant
units based upon this and other physical constants.
The relation (Planck relation) of a photon's energy (E) to
its frequency (f) is:
E = h × f
The constant and the Planck function were conceived by Max
Planck as a mathematical means to describe the black-body spectrum.
The reduced Planck constant (aka Dirac constant,
abbreviated ℏ) is h / 2π,
which is convenient because many equations
incorporate this expression.