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
E = (6.626176 x 10-34) f
f = E / (6.626176 x 10-34)
See a Khan Academy video, Introduction to light: