# Planck constant

The constant factor which relates the energy of a photon to its frequency. Symbol, *h*. According to the 2014
CODATA recommended values, it is 6.626 070 040 × 10⁻³⁴
joule-seconds, with a standard uncertainty of 0.000 000 081 × 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 1.054 571 800 x 10⁻³⁴ J s.

## Setting a fixed value for the Planck constant

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.

## Replacing the International Prototype of the Kilogram with *h*

The Planck constant is given in joule seconds. The joule, a unit of energy, is

.

Copyright © 2000 Sizes, Inc. All rights reserved.

Last revised: 26 November 2017.