The effective depth of penetration of an electromagnetic wave in a conductive medium. The skin depth is the distance in which the wave decays to 1/e (about 37%) of its value; it can be expressed as:

δ_{s} = (2/σμω)^{1/2} = (2/σ)(ε/μ)^{1/2},

where
δ_{s} = skin depth
σ = electrical conductivity
ω = 2πf = angular frequency in radians/s f = frequency in Hz
μ = μ_{r}μ_{0} = magnetic permeability
μ_{r} = relative magnetic permeability of the conductor
μ_{0} = relative magnetic permeability of free space = 4π × 10^{−7} newton per ampere squared (N/A^{2})
ε = ε_{r}ε_{0} = dielectric permittivity
ε_{r} = relative dielectric permittivity of the material
ε_{0} = dielectric permittivity of free space = 8.854 × 10^{−12} farads per meter (F/m).