Planck's constant, symbolized *h*, relates the energy in one quantum (photon) of electromagnetic radiation to the frequency of that radiation.

In the International System of units (SI), the constant is equal to approximately 6.626176 x 10^-34 joule-seconds. In the centimeter-gram-second (cgs) or small-unit metric system, it is equal to approximately 6.626176 x 10^-27 erg-seconds.

The energy *E* contained in a photon, which represents the smallest possible “packet” of energy in an electromagnetic wave, is directly proportional to the frequency *f* according to the following equation:

*E* = *hf*

If *E* is given in joules and *f* is given in hertz (the unit measure of frequency), then:

*E* = (6.626176 x 10^-34) *f*

and conversely:

*f* = *E* / (6.626176 x 10^-34)