In physics, **Planck's constant**, named after the physicist Max Planck, is a fundamental value equal to:

*h*= 6.6261 × 10^{-34}Js

appearing in all the equations of quantum mechanics. It can be seen as a conversion factor between frequency and energy, especially for photons. The unicode symbol ℎ represents Planck's constant. Sometimes the abbreviation

*h_bar*=*h*/ 2π

is used, where π is Archimedes' constant, the ratio of the circumference of a circle to its diameter. h_bar is a lower-case h with a line through it, the symbol ℏ.

One of the values that can be derived from Planck's constant is the Planck length, 1.6 × 10^{-35}m, which is the smallest meaningful length in quantum mechanics; any two points separated by less than the Planck length are indistinguishable from each other. Similarly, the amount of time it takes a photon to travel one Planck length is Planck time: 10^{-43} seconds. This is the smallest meaningful division of time.

The planck length and planck time are used as the fundamental units in the system of Planck units.