Planck's constant or the Planck constant is the proportionality constant relating a photon's energy to its frequency. The constant is the basis for the definition of the kilogram unit of mass and is important in the field of quantum mechanics.

Planck's constant is expressed by the symbol **h**

h = 6.62606896(33) x 10^{-34} J·sec

h = 4.13566733(10) x 10^{−15} eV·sec

Because the constant's value depends on the units used to describe it, it is known exactly in atomic units, but "only" out to 12 parts per billion using the metric system.

### Sources

- Bowley, R.; Sánchez, M. (1999). Introductory Statistical Mechanics (2nd ed.). Oxford: Clarendon Press. ISBN 978-0-19-850576-1
- Schlamminger, S.; Haddad, D.; Seifert, F.; Chao, L. S.; Newell, D. B.; Liu, R.; Steiner, R. L.; Pratt, J. R. (2014). "Determination of the Planck constant using a watt balance with a superconducting magnet system at the National Institute of Standards and Technology".
*Metrologia*. 51 (2): S15. arXiv:1401.8160. doi:10.1088/0026-1394/51/2/S15