The constant factor which relates the energy of a photon to its frequency. Symbol, *h*. According to the 2006
CODATA recommended values, it
is 6.626 068 96 × 10^{−34} joule-seconds, with a standard
uncertainty of 0.000 000 33 × 10^{−34}.

It is named for the German physicist Max Planck.

It is often encountered as the reduced Planck constant, symbol *ħ*
(pronounced “h bar”), which is h divided by 2 pi, or 1.054 571 628 x 10^{-34}
J s.

At its 24^{th} meeting (Paris, October 2011), the CGPM decided to declare in advance its intention to
make the value of the Planck constant a matter of definition, rather than
something to be determined experimentally. The new value will be *exactly*
6.626 06X × 10^{−34 }joule-seconds, where X stands for one or more
yet to be determined digits. The new definition will not be adopted before 2014.

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Last revised:
26 October 2011.