The Planck impedance is directly coupled to the impedance of free space, Z, and differs in value from Z only by a factor of 4π.

The Planck impedance is the unit of electrical resistance, denoted by Z, in the system of natural units known as Planck units.

This sets the Planck impedance, Z equal to Z/4π, where Z is the characteristic impedance of free space.

If the Planck charge were instead defined to normalize the permittivity of free space, ε, rather than the Coulomb constant, 1/(4πε), then the Planck impedance would be identical to the characteristic impedance of free space.